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.

docker/Makefile

@@ -7,3 +7,9 @@ all: $(OBJECTS)
 
 %/Dockerfile.alpine: Dockerfile.j2 render_template
 	./render_template "$<" "{\"target_file\":\"$@\"}" > "$@"
+
+%/Dockerfile.buildx: Dockerfile.j2 render_template
+	./render_template "$<" "{\"target_file\":\"$@\"}" > "$@"
+
+%/Dockerfile.buildx.alpine: Dockerfile.j2 render_template
+	./render_template "$<" "{\"target_file\":\"$@\"}" > "$@"