feat: build and push new prebuilt docker image

[sammcj]

Summary of changes

Fix for #163

• Build, label, tag and publish Docker image to Githubs image repository.
• Simplify Action's Dockerfile to pull the built image.
• (Fixes auth issues faced last PR! feat: pre-build dockerfile #201)

Do any of the followings changes break current behaviour or configuration?

• No

How changes have been tested

• I've forked the repo and done the build and publish.
• Tested I'm able to pull the built image locally.

List any unknowns

• I haven't tested the changes in this repo.
  • I don't know what package permissions @anothrNick has setup on this repo, if it's default I believe public repos have public packages.

Once merged, the Action should be tested to ensure it's operating the same.