docker: docker.py make --no-cache skip checksum test

If you invoke with NOCACHE=1 we pass --no-cache in the argv to
docker.py but may still not force a rebuild if the dockerfile checksum
hasn't changed. By testing for its presence we can force builds
without having to manually remove the docker image.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20170725133425.436-5-alex.bennee@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>

tests/docker/docker.py

@@ -263,7 +263,8 @@ class BuildCommand(SubCommand):
         tag = args.tag
 
         dkr = Docker()
-        if dkr.image_matches_dockerfile(tag, dockerfile):
+        if "--no-cache" not in argv and \
+           dkr.image_matches_dockerfile(tag, dockerfile):
             if not args.quiet:
                 print "Image is up to date."
         else: