github actions + multiplatform support + nyx

[Ilshidur]

Hello.

Thank you for this PR. I have been late to respond, please accept my apologies.

Concerning your contribution :

• GitHub Actions are a nice touch but I would need to tweak those a little to fit the current versioning schema of the Docker image. I agree that multiplatform is a must nowadays.
• As the goal of this Docker image is to be very lightweight, I believe adding Nyx is counter-productive, even though I can imagine it would be very useful.
• Setting /etc/apk/repositories on build is cool, although, wouldn't it require apk updateing before the apk add step in order to gather the version on the edge repositories ?

[adriadam10]

I will change the name of my branch to tor-relay-docker-nyx to difference it. In relation with the repositories, i think that are updated before a package is downloaded, but for precaution i will add the apk update command