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A .dockerignore file keeps unnecessary files out of a Docker build context before that context is sent to the builder. That can cut needless transfers—especially when you use a remote builder—and prevent local files from being available to build instructions. It does not automatically make the final image smaller, and it is not a substitute for proper build-secret handling.
What does a .dockerignore file do?
A Docker build uses a context: the set of files available to the build. For a filesystem context, Docker processes directories recursively. Build instructions such as COPY and ADD can use files from that context. A .dockerignore file filters matching paths before the context is sent to the builder. Docker describes this as a way to exclude files or directories from the build context and avoid sending unwanted material to the builder. Docker Docs: Build context.
That boundary is the practical reason to maintain the file: it determines which local paths are available to build instructions. If a path is excluded, a later instruction that expects it may fail. Conversely, files that are not needed can add work when Docker prepares and transfers the context.
Why can a Docker build context be too large?
The context may include much more than the files required to produce an image: version-control data, generated output, local dependencies, caches, or other workspace files. Docker recommends excluding irrelevant material; its examples include .git, build artifacts, and package-manager directories such as node_modules. Docker Docs: Building best practices.
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Reducing the context can avoid unnecessary transfer and improve build speed, particularly with a remote builder. Docker Build Cloud states that files matched by ignore rules are not transferred to the remote builder. The size of any speed improvement depends on the context and build workflow; Docker does not establish a universal time or percentage gain. Docker Docs: Build Cloud optimization.
Do not confuse a smaller context with a smaller final image. .dockerignore filters build input; the files that end up in an image depend on what the Dockerfile’s instructions place there.
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What should you put in .dockerignore?
Start with the files and directories that the build does not need, then check every Dockerfile instruction and build script against that list. Common candidates include:
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- Local dependency directories such as
node_modules, if the build installs or prepares its own dependencies. - Local configuration or credential files that should not be sent as ordinary context files.
These are not universal exclusions. For example, excluding .git means build steps cannot run Git commands such as git rev-parse against the repository data in the context. Likewise, excluding a generated directory breaks a build if a later COPY or script expects it. Docker Docs: Building best practices.
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How do ignore patterns and exceptions work?
Patterns are newline-separated and similar to Unix shell globs. Docker disregards leading and trailing slashes, and ** matches any number of directories, including none. A comment begins with # in column 1. The historical pattern . is ignored. Docker Docs: Build context.
A leading ! creates an exception to an exclusion. Rules are evaluated in order, and the last matching rule determines whether a path is included or excluded. Put exceptions and later exclusions in an order that expresses the intended result; a subsequent matching rule can reverse an earlier one.
node_modules
!node_modules/needed-package
node_modules/needed-package/tempIn this example, the first rule excludes the dependency directory, the second re-includes the named package, and the final rule excludes its temp path again. Adapt patterns to the actual paths your build needs.
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For a standard build, place .dockerignore at the root of the build context—not necessarily the repository root if the command uses a different context directory. When separate Dockerfiles need different exclusions, Docker also supports Dockerfile-specific ignore files placed with the corresponding Dockerfile, for example build.Dockerfile.dockerignore. When both apply, the Dockerfile-specific file takes precedence over the context-root .dockerignore. Confirm that the context path, Dockerfile, and ignore-file location match the build command you actually run. Docker Docs: Build context.
Does .dockerignore protect secrets?
It can keep a local file such as .env or a credential file out of the build context, which is a useful additional filter. It is not a complete secret-management mechanism: it does not prevent every possible leak, and it should not be used to pass secrets to build steps.
Docker says build arguments and environment variables are inappropriate for passing secrets because they persist in the final image. Use BuildKit secret mounts or SSH mounts to make a secret available to the relevant build instruction instead. Docker Docs: Build secrets. Keep local secret files excluded as appropriate, but treat secret mounts as the mechanism for supplying build secrets.
Why can’t my build find a file after I added it to .dockerignore?
The most likely explanation is that an ignore rule matches the missing path. Check the pattern itself, any matching exception, and the order of all rules: the last matching rule decides the result. Also verify that you edited the ignore file used by the command. A Dockerfile-specific ignore file takes precedence over the context-root file when both apply.
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If the file is intentionally excluded, remove or revise the applicable rule only after confirming the build needs that file. If you excluded .git, for instance, Git-dependent build steps will not be able to access repository metadata from the context. Dockerfile and .dockerignore files are still sent to the builder because they are needed to run the build, but they cannot be copied into the image with ADD, COPY, or bind mounts. Docker Docs: Build context.
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