docker: add a Makefile trick to auto-complete implicit rules

Bash completion figures out the targets from the Makefile, but ignores
implicit/pattern rules. Add some magic to create real docker-test.* and
docker-html.* targets to get proper autocompletion.

docker/Makefile.local

@@ -23,8 +23,8 @@ docker-html.%: hawkmoth-test.% FORCE
 	mkdir -p doc/_build
 	docker run $(DOCKER_TEST_OUT_MOUNT) $< make BUILDDIR=/out html
 
-docker-test: docker-test.debian-bullseye FORCE
-docker-html: docker-test-html.debian-bullseye FORCE
+# Generate targets for auto-completion
+$(shell find $(docker_dir) -name 'Dockerfile.*' | sed 's/.*\.\(.*\)/docker-test.\1 docker-html.\1/'):
 
 FORCE: