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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Docker Compose can mount a file-backed secret into a service at /run/secrets/<secret_name>, once you declare the secret at the top level and grant it to that service. Your application then reads the file. The local fallback is not a Docker feature. It is a few lines of application code or configuration, and it needs explicit rules so it never hides a missing production secret.
The pattern in one picture
- Declare the secret in the Compose file and grant it to the services that need it.
- Docker mounts it read-only at
/run/secrets/<name>. - The app reads a configured path first. Only in an explicit development mode does it try a local-only file.
- If nothing is found, the app fails loudly.
Docker documents steps 1 and 2. Steps 3 and 4 are an implementation recommendation built on that documented behavior. Docker does not define a fallback path or precedence rules.
Step 1: Declare and grant the secret in Compose
services:
app:
image: my-app:dev
secrets:
- db_password
environment:
APP_ENV: development
DB_PASSWORD_FILE: /run/secrets/db_password
secrets:
db_password:
file: ./secrets/db_password.txt
Two details matter here:
- A top-level
secretsentry on its own grants nothing. A service gets the secret only if its ownsecretslist names it. - The short syntax mounts the file at
/run/secrets/<secret_name>. Long syntax lets you choose a different target name or an absolute target path.
The source can be a host file, as above, or, in Docker Compose, an environment variable. With a file source, Compose uses the file’s contents and bind-mounts the file into the container.
Keep the local file out of version control:
# .gitignore
secrets/
Step 2: Read the secret with explicit precedence
This Python example is illustrative. The same logic ports to any language.
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import os
from pathlib import Path
def read_secret(name: str) -> str:
primary = os.environ.get(f"{name.upper()}_FILE", f"/run/secrets/{name}")
candidates = [primary]
# The local file is considered only when development mode is explicit.
if os.environ.get("APP_ENV") == "development":
candidates.append(f"./secrets/{name}.txt")
for path in candidates:
p = Path(path)
if p.is_file():
return p.read_text().strip()
raise RuntimeError(
f"Secret '{name}' not found. Tried: {', '.join(candidates)}"
)
Design choices worth keeping, whatever your language:
- Mounted path wins. If a runtime secret is present, it takes precedence over any local file.
- The fallback is opt-in. It is enabled by an explicit development setting, not by the mere absence of a file. A production container that lost its secret should crash, not quietly use a stale developer credential.
- Unreadable or missing means failure. The error names the paths tried but never prints secret contents.
- Document it. Write down which source wins and how failures behave, and test both branches.
Does your image already support _FILE?
Some images accept variables such as MYSQL_ROOT_PASSWORD_FILE, which point at a file instead of holding the value. Docker describes this as a convention supported by certain images, including Docker Official Images such as MySQL and Postgres. It is not a universal rule. Check the image’s documentation. If the image does not support it, your application must read the file itself, as in the example above.
Docker also advises against passing sensitive values as plain environment variables, since they can be visible to processes and show up in logs. Passing a file path in an environment variable is the safe variant of this idea.
Limitations of local Compose secrets
- Linux containers only. Docker states that Compose supports secrets only for Linux containers. Windows containers support bind-mounting directories only.
- Permission settings are ignored. A file-backed secret is a bind mount, and the
uid,gidandmodesettings are silently ignored for file sources. Do not rely on them to tighten permissions. Protect the host file itself. - It is not encrypted storage. Docker documents the file source as a bind mount. The encryption guarantees Docker describes for Swarm do not carry over to local Compose.
Trust the Compose project you run
Docker’s trust-model guidance warns that a Compose file can control how containers interact with the host. File-reference fields, including file-backed secrets, can read host files available to the user running Compose, including through symlinks. Their contents may be read during configuration loading, before any container starts. Only run Compose configuration you trust, and review file references, included files and related options in third-party projects first.
Compose, Swarm and BuildKit secrets are different things
All three use /run/secrets as a default location, which is why they get confused.
| Aspect | Compose file-backed secret | Swarm service secret | BuildKit build secret |
|---|---|---|---|
| Purpose | Runtime file for a service | Runtime file for a Swarm service | Credential for a build step only |
| Source | Host file (or environment variable in Docker Compose) | Swarm-managed secret | File or environment variable |
| Delivery | Read-only bind mount, granted per service | In-memory filesystem mount for the running task | Mount in the build container, default /run/secrets/<id>, custom targets allowed |
| Encryption claims | None documented; it is a bind mount | Mutual TLS in transit, encrypted in the Raft log | Not a runtime mechanism |
| Availability | Linux containers | Swarm services only, not standalone containers | Image builds |
Swarm specifics that don’t apply locally
- On Linux the default mount is
/run/secrets/<name>. Windows uses a different default path. - When the task stops, Docker says the decrypted mount is removed and flushed from node memory.
- A disconnected node’s running task keeps access, but it cannot receive updates until it reconnects.
- Docker documents a 500 KB maximum per secret.
- A secret cannot be removed while a running service uses it. Docker points to versioned secret names and a rotation procedure.
Because your app only reads a file path, the same code works under Compose and Swarm. The security properties behind that path differ.
Build-time credentials
If a build step needs a credential, use a BuildKit secret mount. Do not put credentials in Dockerfile ARG or ENV. Docker’s build checks explain that these can persist in the final image or its metadata. A build mount is not what a running service reads later.
Quick Recap
Checklist
- Secret declared at top level and granted only to services that need it.
- Local secret file git-ignored and readable only by you on the host.
- App reads the mounted path first; local fallback is enabled only by an explicit development setting.
- Missing or unreadable secrets fail startup with a clear, non-leaking error.
- Image
_FILEsupport verified, not assumed. - Third-party Compose files reviewed before running.
- Build-time credentials use BuildKit secret mounts, never
ARGorENV.
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