gitlab: Extract build stages to stages.yml

Extract the build stages used by our job templates to a new file
(stages.yml) to be able to include it with the other templates,
without having to run all the jobs included in the default
.gitlab-ci.yml, which are mainly useful for mainstream CI.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210519185504.2198573-7-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.gitlab-ci.d/stages.yml

+# Currently we have two build stages after our containers are built:
+#  - build (for traditional build and test or first stage build)
+#  - test (for test stages, using build artefacts from a build stage)
+stages:
+  - containers
+  - containers-layer2
+  - build
+  - test

.gitlab-ci.yml

-# Currently we have two build stages after our containers are built:
-#  - build (for traditional build and test or first stage build)
-#  - test (for test stages, using build artefacts from a build stage)
-stages:
-  - containers
-  - containers-layer2
-  - build
-  - test
-
 include:
+  - local: '/.gitlab-ci.d/stages.yml'
   - local: '/.gitlab-ci.d/edk2.yml'
   - local: '/.gitlab-ci.d/opensbi.yml'
   - local: '/.gitlab-ci.d/containers.yml'