Change packages' CI setup to use Docker images

[fingolfin]

(This is a continuation of PR #1732, or actually it's exactly what I wanted in that PR, but which I did not communicate well enough.)

Right now, most packages here on GitHub perform their tests by first compiling a version of GAP (typically from the master branch). That then often means that the tests for 3 minutes compile GAP and essential packages; and then the actual package tests run for 20 seconds or so. That's unsatisfying. It also feels bad to put such a burden on Travis.

To improved this, I'd like to try to use the docker images from

• https://github.com/gap-system/gap-docker-master
• https://github.com/gap-system/gap-docker-stable

for this instead; they are on DockerHub at

• https://hub.docker.com/r/gapsystem/gap-docker-master/
• https://hub.docker.com/r/gapsystem/gap-docker-stable/

The idea now is to change 1-2 packages which us the "standard" Travis setup we made recently, to use Docker instead of compiling GAP. Ideally they should run against both master and stable versions of GAP. Once that works, and if it actually turns out to result in a performance gain, we can then proceed to toll this out to other packages.

Note: If packages start to want to support HPC-GAP and/or 32bit supporting we should consider also providing those via Docker. In order to avoid an explosion of configurations, we could perhaps use the out-of-tree build support to have a single Docker image for master have all four variation in one docker image. Not sure if that makes sense, though, I have not look at all how our Docker images are produced.

[olexandr-konovalov]

I will do this - someone else was complaining elsewhere just recently about the long testmaing cycle on slack, and this may be an alternative. One will lose the ability to test package with the last minute changes, since containers are updated once a day, but will be able to test package with both stable and master branches, and maybe with different compilers, if we will provide them in the container.

[fingolfin]

This would be really nice; quite some packages now spend several minutes in their tests to download and compile GAP and some packages, only to run 30 seconds of actual tests.