Minimal ci workflow

[jtxa]

Here's a first version of a workflow to automatically build on pushes and PRs. There is a lot of room for improvements (I'll open an issue for discussion and ideas). But for an immediate benefit I suggest to merge this soon and do the rest later.

I had a very hard time getting the tests running. At the end it was because of the $SHELL being somehow incompatible.

[jtxa]

Please have a look at build result in my fork to see the result of the workflow.

[sierrafoxtrot]

This is fantastic Josef! I've been beating my head against a much, much uglier attempt at the same thing on and off for a couple of weeks now.