Add action to build and push docker to ghcr.io

[bkeepers]

This adds a GitHub Action to build and publish to ghcr.io on push so users easily access a pre-built docker image.

$ docker run -it --rm \
  -v /tiledata:/data \
  ghcr.io/felt/tippecanoe:latest \
  tippecanoe --output=/data/output.mbtiles /data/example.geojson

It should automatically tag the releases, e.g.

• latest release: ghcr.io/felt/tippecanoe:latest
• major versions: ghcr.io/felt/tippecanoe:2
• PRs: ghcr.io/felt/tippecanoe:pr-348
• branches ghcr.io/felt/tippecanoe:branchname

Containers will also show up in the sidebar by releases. You can see what the first build looks like on my fork: https://github.com/bkeepers/tippecanoe/pkgs/container/tippecanoe

[e-n-f]

Thanks for the contribution. I am wary of approving because I don't understand the full implications of using:

password: ${{ secrets.GITHUB_TOKEN }}

Would you mind linking to documentation about how authentication works on this docker repository? Thanks.

[bkeepers]

No problem, here are overall docs on Publishing docker images, and here are docs on GITHUB_TOKEN secret.

Before each job begins, GitHub fetches an installation access token for the job. The GITHUB_TOKEN expires when a job finishes or after a maximum of 24 hours.
…
People with admin permissions to an enterprise, organization, or repository, can set the default permissions to be either permissive or restricted.

[simon04]

Related PRs: #282, #317
Related issue: #167