Optimize Docker build to avoid rebuilding x86 layers twice

[missinglink]

The current build script builds x86 layers twice:

Once with docker build for -classic tags (single-arch for old Docker/k8s)
Again with docker buildx for multi-arch tags (amd64 + arm64)
This doubles the build time for x86 layers since buildx doesn't reuse the classic build's cache by default.

Solution

Add --cache-from flag to the docker buildx command to reuse layers from the classic build that was just pushed to the registry.

This way: x86 layers are built once (in classic build) buildx reuses those cached layers.
Only arm64 layers need to be built fresh

Implementation

Change build-docker-images.sh to include:
--cache-from="type=registry,ref=$FIRST_REGISTRY/$tag-classic"

This should significantly reduce build times, especially for large images.

[missinglink]

Example changed based on #16

diff --git a/build-docker-images.sh b/build-docker-images.sh
index 7f4085e..11151f8 100644
--- a/build-docker-images.sh
+++ b/build-docker-images.sh
@@ -85,7 +85,11 @@ for tag in ${tags[@]}; do
   done
 
   # newer multi-arch builds using `docker buildx`
+  # use the first registry's classic build as cache source to avoid rebuilding x86 layers
+  FIRST_REGISTRY="${REGISTRIES%% *}"
   docker buildx create --use --platform="$DOCKER_BUILD_PLATFORMS" --name 'multi-platform-builder'
   docker buildx inspect --bootstrap
-  docker buildx build --push --platform="$DOCKER_BUILD_PLATFORMS" $(for registry in $REGISTRIES; do echo "-t $registry/$tag"; done) .
+  docker buildx build --push --platform="$DOCKER_BUILD_PLATFORMS" \
+    --cache-from="type=registry,ref=$FIRST_REGISTRY/$tag-classic" \
+    $(for registry in $REGISTRIES; do echo "-t $registry/$tag"; done) .
 done