How to Use Pulled Image as a Cache with `docker buildx`

Question

We're building Docker images in our CI, but we can't get docker buildx build to utilise a pulled image as a cache.

Here are the docker commands that are executed:

docker pull "ghcr.io/foo/bar/baz"
docker buildx build . --tag "ghcr.io/foo/bar/baz"
docker push "ghcr.io/foo/bar/baz"

How can we modify docker buildx build to use the pulled image as a cache, ensuring that not every RUN is executed if the command remains unchanged?

Here's our Dockerfile:

FROM amd64/alpine:3.16 as build0

RUN apk update && apk add autoconf bash [...]
FROM build0 as build1

RUN mkdir /tmp/build-deps && cd /tmp/build-deps && [...]

FROM build1 as build2
RUN cd /tmp/build-deps && wget ${patch_uri} && [...]

FROM build2 as build3
RUN mkdir /root/build-deps && [...]

FROM build3 as build4
RUN cd /tmp/build-deps/ && mkdir php-ext && cd php-ext && [...]

Answer 1

Use the --cache-from option when building with docker buildx

docker pull "ghcr.io/foo/bar/baz"
docker buildx build . --tag "ghcr.io/foo/bar/baz" --cache-from="ghcr.io/foo/bar/baz"
docker push "ghcr.io/foo/bar/baz"

By specifying this option, Docker will re-use layers from the pulled image where the initial steps have not changed in the Dockerfile and the layers are the same.