ci: Skip check-only container jobs for pre-merge

For pre-merge pipelines, we know that we don't need to run the
container-check jobs unless image-tags.yml has been modified. In this
case, we can just skip the container jobs, because we 'know' that the
containers are present in the upstream repo.

For downstream or off-main pipelines, we still need to run the check
job, because the changed-file tracking is not reliable on other
branches, and because we may need to copy the container image from the
upstream container storage into the user's namespace.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35203>

.gitlab-ci/build/gitlab-ci.yml

@@ -1,6 +1,6 @@
 # Shared between windows and Linux
 .build-common:
-  extends: .container+build-rules
+  extends: .build-rules
   # Cancel job if a newer commit is pushed to the same branch
   interruptible: true
   variables: