Build cache invalidation

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• General rules
• WORKDIR and SOURCE_DATE_EPOCH
• RUN instructions
• Build secrets

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When building an image, Docker steps through the instructions in your Dockerfile, executing each in the order specified. For each instruction, the builder checks whether it can reuse the instruction from the build cache.

General rules

The basic rules of build cache invalidation are as follows:

• The builder begins by checking if the base image is already cached. Each subsequent instruction is compared against the cached layers. If no cached layer matches the instruction exactly, the cache is invalidated.

• In most cases, comparing the Dockerfile instruction with the corresponding cached layer is sufficient. However, some instructions require additional checks and explanations.

• For the ADD and COPY instructions, and for RUN instructions with bind mounts (RUN --mount=type=bind), the builder calculates a cache checksum from file metadata to determine whether cache is valid. During cache lookup, cache is invalidated if the file metadata has changed for any of the files involved.

  The modification time of a file (mtime) is not taken into account when calculating the cache checksum. If only the mtime of the copied files have changed, the cache is not invalidated.

• Aside from the ADD and COPY commands, cache checking doesn't look at the files in the container to determine a cache match. For example, when processing a RUN apt-get -y update command the files updated in the container aren't examined to determine if a cache hit exists. In that case just the command string itself is used to find a match.

Once the cache is invalidated, all subsequent Dockerfile commands generate new images and the cache isn't used.

If your build contains several layers and you want to ensure the build cache is reusable, order the instructions from less frequently changed to more frequently changed where possible.

WORKDIR and SOURCE_DATE_EPOCH

The WORKDIR instruction respects the SOURCE_DATE_EPOCH build argument when determining cache validity. Changing SOURCE_DATE_EPOCH between builds invalidates the cache for WORKDIR and all subsequent instructions.

SOURCE_DATE_EPOCH sets timestamps for files created during the build. If you set this to a dynamic value like a Git commit timestamp, the cache breaks with each commit. This is expected behavior when tracking build provenance.

For reproducible builds without frequent cache invalidation, use a fixed timestamp:

$ docker build --build-arg SOURCE_DATE_EPOCH=0 .

RUN instructions

The cache for RUN instructions isn't invalidated automatically between builds. Suppose you have a step in your Dockerfile to install curl:

FROM alpine:{{% param "example_alpine_version" %}} AS install
RUN apk add curl

This doesn't mean that the version of curl in your image is always up-to-date. Rebuilding the image one week later will still get you the same packages as before. To force a re-execution of the RUN instruction, you can:

• Make sure that a layer before it has changed
• Clear the build cache ahead of the build using docker builder prune
• Use the --no-cache or --no-cache-filter options

The --no-cache-filter option lets you specify a specific build stage to invalidate the cache for:

$ docker build --no-cache-filter install .

Build secrets

The contents of build secrets are not part of the build cache. Changing the value of a secret doesn't result in cache invalidation.

If you want to force cache invalidation after changing a secret value, you can pass a build argument with an arbitrary value that you also change when changing the secret. Build arguments do result in cache invalidation.

FROM alpine
ARG CACHEBUST
RUN --mount=type=secret,id=TOKEN,env=TOKEN \
    some-command ...

$ TOKEN="tkn_pat123456" docker build --secret id=TOKEN --build-arg CACHEBUST=1 .

Properties of secrets such as IDs and mount paths do participate in the cache checksum, and result in cache invalidation if changed.