Improve tests with docker #977

Hi all!

We have been facing many issues related to vim/python versions.

I've been thinking that we need better way to test those combinations (vim + python) in a more "clean and isolated environment". Manually or not.

I thought about using docker to create such environments.

A possibility is to:

• start a docker container
• install the python version we want
• build vim considering this specific python version
• run the tests we want to run
• enable the (python-mode) "developer" to go into the container and see if everything is OK.

I would like to have some feedback from the community regarding this topic and to see if there is anyone to help with it. I would love to do such improvements, but I cannot guarantee that I'll be able to do it in a reasonable time schedule.