Set up GitHub actions for CI

[pmeinhardt]

Maybe we can toss out our Travis CI configuration and replace it with GitHub actions.

We tried migrating from Travis to Circle before, but couldn't dynamically start Docker containers without hassle.
It looks like mint for instance, also uses Docker with GitHub actions:
https://github.com/elixir-mint/mint/blob/v1.2.0/.github/workflows/main.yml

So maybe that's a way forward. 🧑‍🔬

[klappradla]

Wow 🤯! I'd never have had expected this to be so straight forward 👏

[pmeinhardt]

Wow 🤯! I'd never have had expected this to be so straight forward 👏

Me neither. I'm happy it went down so smoothly. 😌