Add official Dockerfile, build official images to GitHub Packages

[jinnatar]

There are 15 unofficial Docker images on DockerHub, so clearly there's demand for an image. All of them are either wildly out of date or do not publish their Dockerfile though, which is sub-optimal.

This PR adds a simple official Dockerfile and automated build action. An example of the automated build and an image it procudes can be seen in the fork where I'm proposing the merge from: https://ghcr.io/artanicus/prometheus-am-executor

Pushes to the master branch will tag the images with latest, all pushes to all branches are tagged with branch_name-6-digit-hash-from-commit and any git tags are tagged as-is. So for example a release tagged as 1.0 would result in an image: ghcr.io/imgix/prometheus-am-executor:1.0

[colakong]

Also, thankyou for putting this together @Artanicus 😸