[RFC PATCH v2 2/3] gitlab: Cache container images

[Fabiano Rosas]

Make use of the --cache-from option from 'docker build' by including
build layers when building and then pulling a previously built image
before the next build.

This was previously done by the docker.py script, but got disabled due
to bad interactions with certain runners. See commit 6ddc3dc7a8
("tests/docker: don't use BUILDKIT in GitLab either").

We now believe those issues to be fixed, so restore the caching
functionality as it brings a significant reduction in container build
times.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 .gitlab-ci.d/container-template.yml | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/.gitlab-ci.d/container-template.yml 
b/.gitlab-ci.d/container-template.yml
index f417452212..31e4e36a7d 100644
--- a/.gitlab-ci.d/container-template.yml
+++ b/.gitlab-ci.d/container-template.yml
@@ -13,7 +13,10 @@
   script:
     - echo "TAG:$TAG"
     - echo "COMMON_TAG:$COMMON_TAG"
-    - docker build --tag $TAG -f "tests/docker/dockerfiles/$NAME.docker" "."
+    - docker pull "$TAG" || true
+    - docker build --tag "$TAG" --cache-from "$TAG"
+      --build-arg BUILDKIT_INLINE_CACHE=1
+      -f "tests/docker/dockerfiles/$NAME.docker" "."
     - docker push "$TAG"
   after_script:
     - docker logout
-- 
2.35.3