docker: fix out-of-tree 'make docker-test-build@debian-powerpc-cross'

Without this patch, it fails with:

IOError: [Errno 2] No such file or directory:
 'tests/docker/dockerfiles/debian-apt-fake.sh'

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <150841324224.31292.4012464539983692364.stgit@bahia.lan>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
diff --git a/tests/docker/Makefile.include b/tests/docker/Makefile.include
index 02cd520..f1a398e 100644
--- a/tests/docker/Makefile.include
+++ b/tests/docker/Makefile.include
@@ -44,7 +44,7 @@
 		$(if $(EXECUTABLE),--include-executable=$(EXECUTABLE)),\
 		"BUILD","$*")
 
-docker-image-debian-powerpc-cross: EXTRA_FILES:=tests/docker/dockerfiles/debian-apt-fake.sh
+docker-image-debian-powerpc-cross: EXTRA_FILES:=$(SRC_PATH)/tests/docker/dockerfiles/debian-apt-fake.sh
 
 # Enforce dependancies for composite images
 docker-image-debian: docker-image-debian9