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Use BuildKit secret mounts for files, tokens, and passwords, and SSH mounts for agent sockets or SSH keys. Supply the credential only when you invoke the build, consume it in the single RUN instruction that needs it, and never place it in ARG, ENV, a copied file, a log, or a cache export. A secret mount exists only for that instruction and is not written into the resulting image when used correctly.
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What a Docker build secret protects
A Docker build often needs a credential to download a private package, read an object from protected storage, authenticate to an internal API, or clone source code. The credential is needed while the image is being assembled, but it should not be needed by the application at runtime. A build secret is the mechanism that keeps that short-lived requirement separate from the image’s filesystem and configuration.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallBuildKit takes a secret from the build client and temporarily exposes it inside the build container for the duration of one build instruction. The value is not included in the layer’s normal cache checksum. That design reduces the chance that a password or token becomes part of an image, but it does not make every use safe automatically: commands can still print the value, copy it elsewhere, write it into an artifact, or leak it through a shared build environment.
There are two main mount types:
- Secret mounts expose a file, or optionally an environment variable, for passwords, API tokens, certificates, and other file- or value-based credentials.
- SSH mounts expose an SSH agent socket or key material for operations such as cloning a private Git repository.
For a remote private Git context, BuildKit also supports pre-flight authentication secrets. Those credentials are used before the Dockerfile runs, while BuildKit fetches the context or source.
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Why ARG, ENV, and COPY are unsafe for secrets
Docker explicitly warns that build arguments and environment variables are inappropriate for passing secrets because they persist in the final image. An ARG value can appear in image metadata, command history, intermediate layers, or build logs. An ENV value is part of the image configuration and can be read by anyone who can inspect or run the image.
Copying a credential file is worse: the file becomes part of a filesystem layer. Deleting it in a later instruction does not erase the earlier layer, and a user who can pull the image may still retrieve it from layer content. A multi-stage build is not a guaranteed repair. If a secret entered an intermediate layer or an exported cache, it can remain available to anyone who obtains that layer or cache even when the final stage does not contain the file.
Keep local credential files outside the build context whenever possible. If a file must remain in the directory used as context, exclude it with .dockerignore. This prevents accidental inclusion by COPY . ., but it does not replace a secret mount: a file that is merely ignored is still present on the developer or CI machine and can be deliberately copied by another instruction.
Prerequisites: use a BuildKit-capable builder
Secret and SSH mounts are BuildKit features. Current Docker installations generally use BuildKit by default, while older engines or custom builders may require explicit configuration. Verify the builder before changing a Dockerfile used by a team or CI system.
- Check the active builder with
docker buildx ls. A builder that supports BuildKit should be listed and selected. - If your workflow uses the classic command, enable BuildKit for that invocation with
DOCKER_BUILDKIT=1 docker build ., or usedocker buildx build. - Pin a suitable Dockerfile frontend at the top of the Dockerfile, for example
# syntax=docker/dockerfile:1. The frontend version controls which mount options are understood. - Make sure the account running the build can read the local secret file or access the SSH agent. The build client, not the Dockerfile, supplies the credential.
Use a builder you trust. BuildKit prevents the secret value from being used as a cache checksum, but a malicious or compromised builder can still inspect data made available to a build instruction.
Safest default: mount a secret file for one instruction
Pass the file at build time
Give the credential a stable, descriptive ID and pass a local file with --secret:
docker build --secret id=aws,src="$HOME/.aws/credentials" .The ID is a label used by the Dockerfile; it does not have to match the filename. The source path is read by the build client and is not sent as an ordinary build-context file.
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Consume it with RUN --mount
Mount that ID only on the command that requires it:
# syntax=docker/dockerfile:1
FROM alpine
RUN --mount=type=secret,id=aws
AWS_SHARED_CREDENTIALS_FILE=/run/secrets/aws
aws s3 cp s3://private-bucket/object /tmp/objectOn Linux, the default destination is /run/secrets/<id>. In this example the file is available as /run/secrets/aws while the command runs. When the instruction finishes, the mount disappears. The downloaded object may remain in /tmp/object, so decide whether that output belongs in the final image or should be consumed and removed in the same instruction.
Do not run a diagnostic such as cat /run/secrets/aws, interpolate the value into an echo, or pass it to a command whose verbose mode prints request headers. Build logs are frequently retained by CI systems and are often readable by more people than the original secret.
Control the mount path, environment form, and permissions
Use target= when software expects another path
If a tool only reads a particular filename, set an explicit target:
RUN --mount=type=secret,id=license,target=/etc/vendor/license.key
vendor-tool --license-file /etc/vendor/license.keyThe target is still temporary. Do not copy it to a permanent configuration directory unless that file is intentionally non-sensitive.
Expose a value as an environment variable only when necessary
Some programs do not accept a file path. Docker supports env= to expose the mounted secret as an environment variable:
docker build --secret id=api_token,env=API_TOKEN .RUN --mount=type=secret,id=api_token,env=API_TOKEN
sh -c 'client --token "$API_TOKEN"'An environment variable is convenient but easier to disclose accidentally through shell tracing, process diagnostics, or a tool’s debug output. Prefer a file when the program supports one, disable shell tracing, and keep the variable scoped to the one command.
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Fail closed with required=true
By default, a missing secret mount can make the instruction fail only when the command tries to use the absent file. Require it explicitly so a misconfigured build cannot silently use an unauthenticated fallback:
RUN --mount=type=secret,id=api_token,required=true
test -s /run/secrets/api_token && client --token-file /run/secrets/api_tokenUnderstand default file ownership and mode
Docker documents default secret-file permissions of mode 0400, user ID 0, and group ID 0. A non-root build user may therefore receive a permission error. Set mount options appropriate to the tool and builder, or run only the specific instruction with a user that can read the file. Avoid broadening permissions more than necessary.
Use SSH mounts for private Git and SSH-based tools
An SSH mount is intended for an SSH agent socket or key-backed SSH operation. It avoids turning a private key into an ordinary file layer and lets the SSH client perform its normal host-key and authentication checks.
Forward an agent
Start an agent, load the key on the build client, and forward the default socket:
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# syntax=docker/dockerfile:1
FROM alpine
RUN apk add --no-cache git openssh-client
RUN --mount=type=ssh
mkdir -p -m 0700 /root/.ssh &&
ssh-keyscan git.example.invalid > /root/.ssh/known_hosts &&
git clone [email protected]:team/private-library.git /opt/private-libraryUse a verified host key rather than blindly accepting a server key. A host-key check protects against connecting to an impostor, while the SSH agent keeps the private key on the client.
Use an explicitly named socket when several identities exist
If the client exposes more than one agent or key, assign an ID and reference it in both places:
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docker buildx build --ssh signing=$SSH_AUTH_SOCK .RUN --mount=type=ssh,id=signing
git clone [email protected]:team/private-library.git /srcDo not copy ~/.ssh into the build context. Do not disable host-key verification merely to make a build pass.
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Authenticate a remote private Git context before the Dockerfile starts
When the build context itself is a private Git URL, the Dockerfile cannot mount a secret early enough to fetch it. BuildKit provides predefined GIT_AUTH_TOKEN and GIT_AUTH_HEADER secrets for this pre-flight step.
GIT_AUTH_TOKEN uses a Git-hosting style x-access-token basic-auth construction. GIT_AUTH_HEADER supplies the raw Authorization header value when the server expects a different scheme. These credentials are used while BuildKit fetches the remote context or an ADD source; they are not a general replacement for a secret mount inside later RUN instructions.
Keep the token in the CI secret store and inject it only into the build command. Confirm that the Git server, URL scheme, and authentication header format match, and avoid printing the full command line when it contains a token.
Compare the available approaches
| Approach | Best use | Exposure duration | Persistence risk | Source |
|---|---|---|---|---|
RUN --mount=type=secret |
Files, API tokens, passwords, certificates | One build instruction | Low when the command does not copy or print the value; leaks remain possible through generated files, logs, or caches | Local file or environment supplied with --secret |
RUN --mount=type=ssh |
Private Git, SSH package servers, signing operations using an agent | One build instruction | Private key stays with the client; cloned source or generated artifacts can still contain sensitive data | SSH agent socket or key forwarded with --ssh |
| Pre-flight Git secrets | Fetching a private remote context or remote ADD source |
Context-fetch phase before Dockerfile execution | Not placed in a normal image layer, but the remote builder and logs remain sensitive | BuildKit predefined GIT_AUTH_TOKEN or GIT_AUTH_HEADER |
ARG or ENV |
Non-sensitive build configuration only | Broad; metadata, process environment, or image configuration | High for credentials because values can persist in history, metadata, layers, and runtime inspection | Command line or Dockerfile |
COPY of a credential file |
Not appropriate for a secret | At least the layer containing the file | High; deleting it later does not remove the earlier layer or exported cache | Build context |
Prevent leaks through layers, history, logs, and cache
Keep the secret out of shell text
BuildKit hides the mounted value from ordinary layer content, but the shell command itself is recorded as part of the instruction. A command such as RUN curl -H "Authorization: Bearer $TOKEN" ... can expose the token if shell expansion, tracing, or a verbose client writes the complete command or request to logs. Read the mounted file directly and use a client option that consumes a filename whenever possible.
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Do not write sensitive outputs into reusable layers
A command can safely read a token and still leak it by generating a configuration file, embedding credentials in a downloaded URL, or writing a package-manager cache that includes authenticated endpoints. Remove temporary output in the same instruction, use tools’ credential-file options, and inspect generated files before they become a layer.
Treat cache exports and shared builders as sensitive
BuildKit excludes secret values from cache checksums, so changing a token does not necessarily invalidate a step. That protects the value from the checksum itself; it does not make a cache harmless. A command that produced a credential-bearing artifact can leave that artifact in a local or remote cache. Restrict who can read exported caches, avoid exporting caches from untrusted projects to a shared registry, and separate builders for workloads with different trust levels.
Understand what image history can and cannot show
A correctly mounted secret should not appear as a file in the final image. However, literal credentials in an ARG, an expanded command, or an ENV value can be visible through image inspection and history. Review both the Dockerfile and the built image; checking only the final filesystem is insufficient.
Design a CI build around least privilege
- Store each credential in the CI system’s encrypted secret store, with a scope limited to the repository, branch, environment, or job that needs it.
- Give it a specific ID such as
package_token,signing_ssh, orcloud_credentials. Avoid one all-purpose token. - Materialize a file only for the build step, or expose an environment value to the Docker client without writing a permanent workspace file.
- Invoke
docker build --secret id=package_token,src=/path/to/temporary/file .ordocker buildx build --ssh default .and mask command output in the CI interface. - Use a narrow
RUN --mountand fail if the credential is absent. Do not make the entire Dockerfile depend on a globally set environment variable. - Restrict cache export and import permissions. A cache shared by unrelated projects is part of the build’s security boundary.
- Delete temporary files after the build and prevent workspace archiving from collecting them.
Use separate read-only credentials for downloading dependencies and write-capable credentials for publishing. A build that only needs to read a private package repository should not receive a credential that can delete packages or modify production systems.
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The Dockerfile reference documents different behavior for Windows containers. A secret is a read-only, single-file bind mount; an explicit target is required. The mode, uid, and gid options are ignored, and the temporary file is restricted by ACLs. A step may need to run as ContainerAdministrator to read it.
Do not carry Linux assumptions such as /run/secrets/<id>, Unix ownership, or permission bits into a Windows Dockerfile. Set a Windows path explicitly, test access under the user configured by the image, and ensure the ACL does not grant broader access than the command needs.
Troubleshoot common failures without exposing the credential
“Unknown flag” or “unsupported mount type”
- Confirm that BuildKit or
buildxis active. - Check the Dockerfile frontend directive and update it to a supported
docker/dockerfilefrontend. - Verify that the selected remote builder supports secret and SSH mounts.
The mounted file is missing
- Ensure the build command’s ID exactly matches
id=inRUN --mount. - Check that the source path exists on the client and is readable by the account launching Docker.
- Add
required=trueso a missing credential fails immediately rather than triggering an unsafe fallback.
Permission denied
- Remember the Linux default of mode
0400, owned by UID 0 and GID 0. - Run only the consuming instruction with an appropriate user, or configure mount ownership supported by your frontend and builder.
- For Windows, use an explicit target and verify the container user’s ACL access.
Private Git authentication fails
- Use an SSH mount for an SSH URL and confirm the agent has the intended identity.
- Use
GIT_AUTH_TOKENorGIT_AUTH_HEADERfor a remote context fetched before Dockerfile execution. - Check host-key verification, repository permissions, URL scheme, and the server’s required authorization format without printing the token.
The build keeps using an old result after a credential change
Secret values are deliberately excluded from cache checksums. Change the non-secret inputs that should determine the result, invalidate the relevant step deliberately, or use an explicit cache policy. Never solve this by putting the token into an ARG; that trades cache behavior for credential exposure.
Respond to an accidental exposure
Assume a credential is compromised if it appeared in an argument, environment variable, copied file, image layer, build log, generated artifact, or shared cache. Do not rely on deleting the image tag or editing the Dockerfile alone.
- Revoke or rotate the credential immediately at the issuing system.
- Invalidate affected sessions, deploy keys, certificates, or package credentials where the provider supports separate revocation.
- Remove exposed images, intermediate layers, logs, and cache exports from registries and CI storage according to their retention controls.
- Inspect image history, layer contents, build logs, cache exports, and workspace archives to determine the exposure window.
- Rebuild with a narrowly scoped secret mount, then verify that the new image and logs contain neither the value nor a credential-bearing artifact.
- Review access logs for use of the old credential and document the rotation for incident response.
A practical pre-release checklist
- BuildKit-capable builder selected and Dockerfile frontend pinned.
- Every credential has a descriptive ID and the smallest possible permission set.
ARG,ENV, andCOPYare used only for non-sensitive data.- Each secret appears in the narrowest possible
RUN --mountinstruction. - SSH operations use
--ssh, verified host keys, and an agent rather than copied private keys. - Remote private contexts use the appropriate pre-flight Git secret.
- Secret files are outside the build context or excluded by
.dockerignore. - Commands do not echo values, enable shell tracing, print authorization headers, or write credentials into generated files.
- Cache exports, shared builders, CI logs, and workspaces are treated as sensitive infrastructure.
- Linux ownership and Windows ACL behavior have been tested under the actual build user.
- Rotation and revocation procedures are documented and tested.
A 2023 arXiv study found secrets in 8.5% of 337,171 Docker Hub images, along with exposed certificates and hosts associated with compromised keys. That figure describes the study’s sample and year, not a universal current rate, but it illustrates why a credential that works during a build must be kept out of the image supply chain.
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