CI: cache PR CI Docker builds

Author: Kobzol

src/ci/docker/run.sh

@@ -92,21 +92,38 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
  # Print docker version
  docker --version
 
- # On non-CI or PR jobs, we don't have permissions to write to the registry cache, so we should
- # not use `docker login` nor caching.
- if [[ "$CI" == "" ]] || [[ "$PR_CI_JOB" == "1" ]];
+ REGISTRY=ghcr.io
+ # PR CI runs on rust-lang, but we want to use the cache from rust-lang-ci
+ REGISTRY_USERNAME=rust-lang-ci
+ # Tag used to push the final Docker image, so that it can be pulled by e.g. rustup
+ IMAGE_TAG=${REGISTRY}/${REGISTRY_USERNAME}/rust-ci:${cksum}
+ # Tag used to cache the Docker build
+ # It seems that it cannot be the same as $IMAGE_TAG, otherwise it overwrites the cache
+ CACHE_IMAGE_TAG=${REGISTRY}/${REGISTRY_USERNAME}/rust-ci-cache:${cksum}
+
+ # On non-CI jobs, we don't do any caching.
+ if [[ "$CI" == "" ]];
  then
  retry docker build --rm -t rust-ci -f "$dockerfile" "$context"
- else
- REGISTRY=ghcr.io
- # Most probably rust-lang-ci, but in general the owner of the repository where CI runs
- REGISTRY_USERNAME=${GITHUB_REPOSITORY_OWNER}
- # Tag used to push the final Docker image, so that it can be pulled by e.g. rustup
- IMAGE_TAG=${REGISTRY}/${REGISTRY_USERNAME}/rust-ci:${cksum}
- # Tag used to cache the Docker build
- # It seems that it cannot be the same as $IMAGE_TAG, otherwise it overwrites the cache
- CACHE_IMAGE_TAG=${REGISTRY}/${REGISTRY_USERNAME}/rust-ci-cache:${cksum}
+ # On PR CI jobs, we don't have permissions to write to the registry cache,
+ # but we can still read from it.
+ elif [[ "$PR_CI_JOB" == "1" ]];
+ then
+ # Enable a new Docker driver so that --cache-from works with a registry backend
+ docker buildx create --use --driver docker-container
 
+ # Build the image using registry caching backend
+ retry docker \
+ buildx \
+ build \
+ --rm \
+ -t rust-ci \
+ -f "$dockerfile" \
+ --cache-from type=registry,ref=${CACHE_IMAGE_TAG} \
+ --output=type=docker \
+ "$context"
+ # On auto/try builds, we can also write to the cache.
+ else
  # Log into the Docker registry, so that we can read/write cache and the final image
  echo ${DOCKER_TOKEN} | docker login ${REGISTRY} \
  --username ${REGISTRY_USERNAME} \