Build a custom image that contains everything installed as part
of each CI run in the before_script.

The Dockerfile contains instructions on how to submit changes to the
image in upstream without causing temporary CI failures.

The image needs to be identified per build, because it pulls things from
internet that will change over time. That means that an image built from
the same Weston commit may be different if built again. Hence the
unique image tag contains also the build id.

To avoid cluttering forks with unnecessary images, the automatic image
build will happen only on a master branch in the upstream repository and
only if Dockerfile is changed. Contributors can trigger an image build
in their own branches by launching a pipeline from the Web UI with
FORCE_DOCKER variable.

Signed-off-by: Pekka Paalanen <pq@iki.fi>