docker: ensure working directory is clean

Using git archive to transfer files to the docker images means
uncommitted modifications aren't transfered. It can be confusing if the
modifications aren't reflected in the results. Fail with a warning
message if the working directory is not clean.

docker/Makefile.local

@@ -8,6 +8,8 @@ HAWKMOTH_VERSION := $(shell cat src/hawkmoth/VERSION)
 HAWKMOTH_ARCHIVE := $(docker_dir)/hawkmoth-$(HAWKMOTH_VERSION).tar.gz
 
 $(HAWKMOTH_ARCHIVE): $(shell git ls-files)
+	@test -z "$(shell git status --porcelain --untracked-files=no)" || \
+		(echo "error: git working directory is not clean" && false)
 	git archive -o $@ HEAD
 
 # Containers for local testing