Optimize release workflow.

- Split Debian and Alpine into different build matrix
  This starts building both Debian and Alpine based images at the same time
- Make use of Docker BuildKit, which improves speed also.
- Use BuildKit caching for Rust Cargo across docker images.
  This prevents downloading the same crates multiple times.
- Use Github Actions Services to start a docker registry, starting it
via the build script sometimes caused issues.
- Updated the Build workflow to use Ubuntu 20.04 which is more close to
the Bullseye Debian release regarding package versions.

hooks/build

@@ -34,12 +34,17 @@ for label in "${LABELS[@]}"; do
     LABEL_ARGS+=(--label "${label}")
 done
 
+# Check if DOCKER_BUILDKIT is set, if so, use the Dockerfile.buildx as template
+if [[ -n "${DOCKER_BUILDKIT}" ]]; then
+    buildx_suffix=.buildx
+fi
+
 set -ex
 
 for arch in "${arches[@]}"; do
     docker build \
            "${LABEL_ARGS[@]}" \
            -t "${DOCKER_REPO}:${DOCKER_TAG}-${arch}" \
-           -f docker/${arch}/Dockerfile${distro_suffix} \
+           -f docker/${arch}/Dockerfile${buildx_suffix}${distro_suffix} \
            .
 done

hooks/push

@@ -10,7 +10,7 @@ join() { local IFS="$1"; shift; echo "$*"; }
 
 set -ex
 
-echo ">>> Starting local Docker registry..."
+echo ">>> Starting local Docker registry when needed..."
 
 # Docker Buildx's `docker-container` driver is needed for multi-platform
 # builds, but it can't access existing images on the Docker host (like the
@@ -28,7 +28,10 @@ echo ">>> Starting local Docker registry..."
 # First check if there already is a registry container running, else skip it.
 # This will only happen either locally or running it via Github Actions
 #
-docker start registry > /dev/null 2>&1 || docker run -d --name registry --network host registry:2 # defaults to port 5000
+if ! timeout 5 bash -c 'cat < /dev/null > /dev/tcp/localhost/5000'; then
+    # defaults to port 5000
+    docker run -d --name registry --network host registry:2
+fi
 
 # Docker Hub sets a `DOCKER_REPO` env var with the format `index.docker.io/user/repo`.
 # Strip the registry portion to construct a local repo path for use in `Dockerfile.buildx`.