Upgrade docker base container to ubuntu 16.04

Upgrade GitLab docker base container to 16.04 since it is already the recommended version, as stated in the GitLab download section: https://about.gitlab.com/downloads/

The lsb-release package is not included anymore and requires a bunch of dependencies:

The following additional packages will be installed:
  dh-python distro-info-data file libexpat1 libffi6 libmagic1 libmpdec2 libpython3-stdlib libpython3.5-minimal libpython3.5-stdlib libsqlite3-0 libssl1.0.0 mime-support python3 python3-minimal python3.5 python3.5-minimal
Suggested packages:
  libdpkg-perl lsb python3-doc python3-tk python3-venv python3.5-venv python3.5-doc binutils binfmt-support
The following NEW packages will be installed:
  dh-python distro-info-data file libexpat1 libffi6 libmagic1 libmpdec2 libpython3-stdlib libpython3.5-minimal libpython3.5-stdlib libsqlite3-0 libssl1.0.0 lsb-release mime-support python3 python3-minimal python3.5 python3.5-minimal
0 upgraded, 18 newly installed, 0 to remove and 5 not upgraded.

Thus I went for the easier approach and just used the /etc/lsb-release file directly.

I'm currently testing it on our on-premise GitLab Instance, it is building and I'm pretty sure it should work (since using it on 16.04 is recommended :) ). But if you wait a few days before merging it, I would report any issues here if I find some.