Github Action for Raspberry Pi builds

[MoJo2600]

I created a Dockerfile to compile welle.io for Raspberry Pi on my x86 machine. I thought I could go a step further and create a github action for it as well, as it is pretty straight forward.

Current state:

• The build works and produces a binary (See: https://github.com/MoJo2600/welle.io/actions/runs/4957542506
• Currently the generated build output is stored as an artifact, but there is no further processing / releasing
• I did not test the binary yet

Not sure if somebody is interessed in this or if you want to integrate it into the welle.io repository. If not, just close this issue / pull request.