Build & push docker on release

[caffeine-addictt]

So I noticed that the instructions for running github-linguist as a docker image requires the end-user to build it locally. Thus for the sake of convenience, I made CI that runs against new tags and pushes to GitHub's ghcr.io docker repository. I have tested these changes on this fork of mine.

Tags:

• buildcache - for build caching (on the max setting)
• latest or prerelease - based on the release tag (should be fine as long as we stick to semver)
• commit SHA

I've also updated README to include pulling images down from ghcr.io.

Cheers, -Alex Ng J. X.

Closes https://github.com/github-linguist/linguist/issues/4613

[lildude]

🤦 I forgot to merge this PR before making the last release.

[Nixinova]

@lildude what about now 😁

[lildude]

Whoops. I forgot again. Lets give it a go now.

[lildude]

Of course I need to manually build and push a container image for the current release. I'll try find time this week.