gitlab-ci: tag docker buildserver image with "stretch"
gitlab-ci: tag docker buildserver image with "stretch"

This should make the migration to bullseye easier because we'll have a
fallback stretch image available as needed.

!1012

f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver