docker: cleanup temp directory after test

There are temp directories named "docker-src.*" after doing docker
tests.  I don't see much point in keeping that (it only contains the
qemu.tar which is exactly current tree, and the copied "run" file).
Let's remove it after test finished.

CC: "Alex Bennée" <alex.bennee@linaro.org>
CC: Fam Zheng <famz@redhat.com>
CC: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20171017071247.32355-1-peterx@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
diff --git a/tests/docker/Makefile.include b/tests/docker/Makefile.include
index ab939f2..38874ec 100644
--- a/tests/docker/Makefile.include
+++ b/tests/docker/Makefile.include
@@ -151,6 +151,8 @@
 			$(IMAGE) 					\
 			/var/tmp/qemu/run 				\
 			$(TEST), "  RUN $(TEST) in ${IMAGE}")
+	$(call quiet-command, rm -r $(DOCKER_SRC_COPY), \
+		"  CLEANUP $(DOCKER_SRC_COPY)")
 
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