Upgrade to Ubuntu:18.04

[pedroasad]

First of all, thank you so much for creating this project, it has been really useful to me so far (in particular, I've been using the themattrix/tox Docker image, which depends on this)!

Now, I'd like to know if you have any plans, or perhaps objections, against upgrading the base image from Ubuntu 16.04 to Ubuntu 18.04. I've already done that in a restricted scenario after copying the repository files into my Python project and using it to build a custom tox image that runs on Ubuntu 18.04, and I'd be happy to submit a pull request if you think it is a good idea.

Details:
In my project, I needed a more recent version of GCC in order to compile my CPython extensions, as part of my testing pipeline, hence the need for 18.04. It seems that some adjustments would need to be done when installing the default python versions from the python-versions.txt provided in tox-base, as pyenv complains about a Missing OpenSSL Lib (I'm not sure what Python version caused this, as the error messages seem to be out of order). Furthermore, some extra packages needed to be installed in tox-base to grant normal gpg operation (I can provide details, if needed). After adding the missing packages to tox-base/Dockerfile and removing all versions except 3.6.6 and 3.7.0 (the ones I need), the build was finally successful.

I am no Docker expert, but I'd be glad to give additional details or to dedicate a few hours into submitting a decent pull request (I guess this would be, like, 1 PR to this project and another one to tox-base).

Cheers,

[bittner]

Upgrading to 18.04 will also make the Docker image significantly smaller (probably ~40M as opposed to 439M currently) as the new Ubuntu images are slim images by default.