Simplify build workflow

[fpiesche]

Got another one for you!

This change makes the GitHub Actions workflow more readable by removing a bunch of the variables, and also fixes building and pushing Docker images - on my original PR this was simply using the GitHub repository owner as the repository name, but as your GitHub user name contains an upper-case D this was failing as Docker repository names must be all lowercase.

If you didn't do this after my previous PR, you will need to add a couple of secrets to this repo (at https://github.com/Didstopia/starbound-server/settings/secrets/actions):

• A secret called GHCR_TOKEN with its value set to a GitHub Personal Access Token for your account (you can create this at https://github.com/settings/tokens/new) with scope permissions set to write:packages (and read:packages which is included in this). I would recommend setting it to never expire as otherwise you'll have to periodically update/re-create it for the workflow to continue working.
• A secret called DOCKERHUB_TOKEN with its value set to a Docker Hub access token; you can create this at https://hub.docker.com/settings/security. The token will need read and write access in order to push new images.