feat: Publish docker image to ghcr

[jphastings]

This PR adds a Github workflow that builds & publishes an OCI/Docker container image to Github's Container Registry (ghcr.io) alongside the executables currently built with Github actions. This PR also adds a brief intro for how to use it in the README, and removes the (now outdated) tools/docker/README.md.

You can see the build running on my own branch (under tag v3.3.8+docker). Try this container out with:

docker pull ghcr.io/jphastings/agate

(This would become ghcr.io/mbrubeck/agate on merge)

I've set up the docker labels so they derive from your git tags. You can select latest, major, minor, or patch versions for auto-update, as well as selecting a specific version too:

# Pulls the most recently released image
# These two are equivalent under most standard setups
docker pull ghcr.io/jphastings/agate
docker pull ghcr.io/jphastings/agate:latest

# Pulls the most recently deployed image within a major version of 3
docker pull ghcr.io/jphastings/agate:3

# Pulls the most recently deployed image within a major version of 3 and minor version of 3
docker pull ghcr.io/jphastings/agate:3.3

# Pulls the most recently deployed image within a major version of 3, minor 3, patch 8
docker pull ghcr.io/jphastings/agate:3.3.8

The specifics of how git tags are pulled into docker tags is detailed here; but the current Agate semver approach will translate well into docker tags without modification.

Do let me know if you have any questions!

Closes #279

[mbrubeck]

Thanks!