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If docker build appears idle before its first Dockerfile instruction, the delay may be Docker preparing or transferring the build context—not a slow RUN command. The context is the set of files available to the build, and a command ending in . makes the current directory that context. Check the transfer output, context path, ignore rules and selected builder before changing Dockerfile steps.
What happens before Dockerfile steps appear?
Docker’s documentation defines the build context as “the set of files that your build can access.” The path, URL or other input supplied to docker build determines that set. With a local directory, Docker may need to prepare and transfer files before reporting Dockerfile instructions.
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A trailing . in a command such as docker build . means the current directory is the context. It does not limit the context to files later referenced by COPY or ADD. A large context can therefore include files the image never uses.
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- Read the output before the first instruction. Look for context loading or transfer status, transferred size and elapsed time. Docker’s documentation shows an example line,
transferring context: 13.16MB 2.2s done. That is an illustration of the output format, not a benchmark or expected threshold. - Check the command’s context argument. Identify the path, URL or other input at the end of the build command. If it is
., inspect the current directory’s contents and size. - Review the builder in use. Docker documents BuildKit as the default for
docker build, except in Windows container mode or whenDOCKER_BUILDKIT=0is set. A custom builder can also affect where the work happens, so verify the active configuration rather than assuming a particular backend. - If the builder is remote, separate upload time from execution. Consider the amount of client-side context being sent and the network connection. A remote builder does not make an oversized context free.
Trim unnecessary files with .dockerignore
For a local directory context, put a .dockerignore file at the context root and list files or directories the build does not need. Common candidates include .git, local build outputs and dependency directories such as node_modules. Do not exclude files that a later COPY or ADD instruction requires.
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Patterns are newline-separated. Docker’s ignore syntax supports ** for matching across directory levels, and when rules negate earlier exclusions, the last matching rule applies. If a Dockerfile-specific ignore file is placed beside a Dockerfile, it takes precedence over the context-root ignore file.
For example, a simple starting point might be:
.git
node_modules
distUse only entries that are irrelevant to your build; the example is not appropriate for every project, and excluding a required input can make a later build step fail.
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Why the selected builder changes what gets transferred
The legacy builder sends the full context to the daemon. BuildKit can transmit files used by the build, skip unused files and incrementally transfer only changed files on repeated builds. Those differences can matter when the context is large, but they do not establish a guaranteed speedup for an individual project.
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When the delay is not the context
If the output shows context transfer completing quickly but the first visible instruction is still delayed, the context alone does not explain the pause. Check which builder is selected and whether the output indicates work occurring before the first reported Dockerfile step. The command, Docker version, builder configuration and build output are needed to diagnose a particular machine; without them, it is not possible to attribute one user’s delay to a specific cause.
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