Allow reusing CI Docker cache when running CI images locally

src/ci/docker/run.sh

@@ -101,10 +101,18 @@ if [ -f "$docker_dir/$image/Dockerfile" ]; then
  # 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.
+ # On non-CI jobs, we try to download a pre-built image from the rust-lang-ci
+ # ghcr.io registry. If it is not possible, we fall back to building the image
+ # locally.
  if ! isCI;
  then
- retry docker build --rm -t rust-ci -f "$dockerfile" "$context"
+ if docker pull "${IMAGE_TAG}"; then
+ echo "Downloaded Docker image from CI"
+ docker tag "${IMAGE_TAG}" rust-ci
+ else
+ echo "Building local Docker image"
+ retry docker build --rm -t rust-ci -f "$dockerfile" "$context"
+ fi
  # 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" ]];