drm_hwcomposer: CI: Use upstream container image conditionally

Right now we use $CI_REGISTRY_IMAGE:latest as the docker image for running build, tidy and checkstyle.

The problem with this is that CI_REGISTRY_IMAGE points to the fork's container registry.

For example, if the ddavenport user has this fork: https://gitlab.freedesktop.org/ddavenport/drm-hwcomposer

Then CI_REGISTRY_IMAGE will be: registry.freedesktop.org/ddavenport/drm-hwcomposer

This won't work for forks that never build the container image.

Use registry.freedesktop.org/drm-hwcomposer/drm-hwcomposer:latest as container image by default.

For MRs that modify .ci/Dockerfile, a container will be rebuild and the forks image will be used instead.

Link: #97 Signed-off-by: Mattijs Korpershoek mkorpershoek@baylibre.com