- fix geckodriver version used following `backend-dev-e2e` base image update
- use Prosody image `prosodyim/prosody:0.12` instead of a deprecated one
- update `pubsub` image with base image update and use of new repos names
Using the `DOCKER_LIBERVIA_REV` environment variable, backend (and related repos) revision
can now be specified. By default (variable empty or not set), the current `tip` version is
used, and `dev` is used for tagging. If the variable is set, the revision will be used and
used for tagging the image.