Refactor & automatically build Docker image

[minus7]

• Refactor Dockerfile
  • Use code from context instead of cloning the repo
  • Download dependencies first for caching
  • Run as unprivileged account (note that this break running on ports <1024 on Docker <20.03; default ports changed accordingly)
• I'm not actually sure the ca-certificates package is required in the image, requesting certificates seems to work without it, though I did not test fully obtaining one.
• Automatically build and deploy Docker image via GitHub Actions
  • I mainly did this so I can use it right away. Dunno if it makes sense, especially since images are on Docker Hub already.

[gbonnefille]

As stated in #293 it is possible to reduce the footprint and increase security, going from scratch.