CI: build and publish Docker image on Github

[lionel-nj]

Summary:

This PR provides support to build and publish a Docker image at each:

• push on the master branch
• pre-release
• release

Expected behavior:

Build and publish a Docker image on the package page of the repository.

Image information is available here

Please make sure these boxes are checked before submitting your pull request - thanks!

• ~[] Run the unit tests with mvn test to make sure you didn't break anything~
• [x] Format the title like "Fix # - " (for example - "Fix #1111 - Check for null value before using field")
• ~[ ] Linked all relevant issues~
• [x] Include screenshot(s) showing how this pull request works and fixes the issue(s)

[barbeau]

Looking at this a bit more, it appears the package for the gtfs-realtime-validator is access-restricted - you can see it shows as "Internal" under the Packages list:

I believe you just need to switch this package to be publicly anonymously accessible - see: https://docs.github.com/en/packages/learn-github-packages/configuring-a-packages-access-control-and-visibility#container-creation-visibility-for-organization-members

[lionel-nj]

Thanks for reviewing @barbeau! I updated the README and changed the privacy settings (I was then able to pull the Docker image successfully).

[lionel-nj]

Thank you @barbeau!