Build and distribute Docker image

[claabs]

Adds the following:

• Dynamic multi-arch support to the Dockerfile (linux/amd64, linux/386, linux/arm64, linux/arm/v7, linux/arm/v6)
• GitHub Actions workflow that automatically builds the Docker image and publishes it to the GitHub Container Registry on this project
• Updates docs to reference prebuilt Docker image
• Minor optimizations to the Dockerfile

Before merging, I believe you'll need to enable the "Improved container support" feature in the GitHub feature preview window.

Once merged, you should see the images appear in the "Packages" section of your repository. You can see an example on my fork.

[nichobi]

Thank you very much, this looks excellent!