Build cache invalidation

Table of contents

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

General rules

The basic rules of build cache invalidation are as follows:

  • Starting with a parent image that's already in the cache, the next instruction is compared against all child images derived from that base image to see if one of them was built using the exact same instruction. If not, the cache is invalidated.

  • In most cases, simply comparing the instruction in the Dockerfile with one of the child images is sufficient. However, certain instructions require more examination and explanation.

  • For the ADD and COPY instructions, the modification time and size file metadata is used to determine whether cache is valid. During cache lookup, cache is invalidated if the file metadata has changed for any of the files involved.

  • 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.

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:3.19 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=foo \
    TOKEN=$(cat /run/secrets/foo) ...
$ TOKEN=verysecret docker build --secret id=foo,env=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.