Nightly debian packages and other GitLab-CI/Docker improvements

This merge requests adds:

1. Nightly Debian Stretch package builds
2. Docker Containers built from the nightly packages on both Debian Stretch and EL7. The resulting containers will provide a better approximation of what an actual release will look like vs the previous lalsuite:nightly container that was constructed from the CI output.
3. Building of packages and Docker containers, on both Debian Stretch and EL7, whenever a tag is pushed to the repository.

Note: The actual packages are currently not made available via a software repository, these will be provided at a later date.

@leo-singer, with your Debian Maintainer's hat on could you please look at the way the Debian packages are built and offer any comments or suggestions.

closes #61 (closed), closes #62 (closed)