Add ci pipeline

I'm sure there is more to add, like flatpak and such, but you will be better than me on this ;)

I created a separate branch, but needed to use latest master as go vet wasn't passing on your master branch. So, better to merge the rest first and see this additional commit here. As you can see, the pipeline is using different stages to control code quality, build and tests (I will look at gitlab code coverage functionality, as I'm more used to github + 3rd parties).

Unfortunately, I needed to publish my own docker image (see the descriptions for more info).