Build and distribute Docker image

[claabs]

• Adds Github Actions to build and push image to Github container registry.
  • When merged, it should just work automatically without any tokens to be setup.
  • You can see how it gets shown on my fork
  • Runs on push to main
• Improve Dockerfile
  • Adds a symlink for stats.json so it can be mounted easier (no need to prime the mount with an empty stats.json)
  • Add deno cache as recommended by the official deno Docker image docs
  • Switch to the alpine base image for a smaller size
• Add Readme section for Docker setup

[claabs]

FYI, I've done 3-4 runs with friends on this image since creation. So it has been tested.