gitlab-ci: Automatically run pipelines for Marge Bot pre-merge only

Marge only merges an MR if the pipeline passed. Running the pipeline
again after merging is redundant.

v2:
* Add rule to ensure docker images are up to date in the main project
  registry (Eric Anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5680>
1 file changed