use wmf-debci docker images
This change replaces generic CI images with wmf-debci images. The
image contains build-essential
and devscripts
so this doesn't need
to be installed in every CI run.
GitLab CI variables WMF_DEBCI_IMAGE
and WMF_DEBCI_VERSION
were
introduced to change the image and/or version tag in a single place.
Bug: T352003