gitlab: limit re-builds of the containers

Most of the time we are just rebuilding the same things. We can skip
this although currently there is no mechanism for picking up new
distro releases.

Rather than try to be too fine grained allow any change to trigger all
the images being rebuilt.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Message-Id: <20200701135652.1366-38-alex.bennee@linaro.org>

.gitlab-ci.d/containers.yml

@@ -17,6 +17,12 @@
     - docker push "$TAG"
   after_script:
     - docker logout
+  rules:
+    - changes:
+      - .gitlab-ci.d/containers.yml
+      - tests/docker/*
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
+    - if: '$CI_COMMIT_REF_NAME == "testing/next"'
 
 amd64-centos7-container:
   <<: *container_job_definition