docker: Clean dangling tarball files

When a container fails, it leaves a dangling tarball which name is
based on a timestamp. Further uses of make won't clean those files,
neither calling the 'docker-clean' target.

Use the .DELETE_ON_ERROR built-in target to let make remove those
temporary tarballs in case of failure.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180818030337.22271-1-f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>

tests/docker/Makefile.include

@@ -25,6 +25,7 @@ IMAGES ?= %
 CUR_TIME := $(shell date +%Y-%m-%d-%H.%M.%S.$$$$)
 DOCKER_SRC_COPY := $(BUILD_DIR)/docker-src.$(CUR_TIME)
 
+.DELETE_ON_ERROR: $(DOCKER_SRC_COPY)
 $(DOCKER_SRC_COPY):
 	@mkdir $@
 	$(if $(SRC_ARCHIVE), \