# Build cache invalidation


When building an image, Docker steps through the instructions in your
Dockerfile, executing each in the order specified. For each instruction, the
[builder](/build/builders/) 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:

```console
$ 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`:

```dockerfile
FROM alpine:3.23 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`](/reference/cli/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:

```console
$ 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.

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

```console
$ 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.