Add custom manager to update BUILDKIT_IMAGE in CNG repository
Relates to gitlab-org/build/CNG!1854 (merged)
Working on a MR in the CNG repository that will add
BUILDKIT_IMAGE: "moby/buildkit:v0.14.0"
to gitlab-ci.yml
This MR will add the relevant custom manager to update the moby/buildkit
image whenever a new one is released.