* Create rolling releases using the Gitlab API.
This commit builds rolling releases of Container images using the latest commit
on master branch if the pipeline passed for it. The script which gets the
references is still un-tested and should be tested.
The latest commit hashes are passed as arguments to the Dockerfile, which is
then used by PIP to install the specific version of the dependency.
* Run as mailman user.
* Run mailman core process as a less-privileged mailman user.
* Add a `docker ps` to travis to know if any container died.
* Spit out more debug information in CI.
* Install su-exec properly.
