WIP: Add Debian build to GitLab CI

Merge !152 (closed) first.

This is currently failing. The build log is pretty cryptic, but the actual reason that it is breaking is that python3-glue cannot be installed within the container. To reproduce:

$ docker run --rm ligo/lalsuite-dev:jessie bash -c 'apt-get -qq update && apt-get -y install python3-glue'
Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 python3-glue : Depends: python3-openssl but it is not going to be installed
                Recommends: python3-xml but it is not installable
E: Unable to correct problems, you have held broken packages.