feat: pre-build dockerfile

[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.

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.

[sammcj]

Note @sbe-arg the build will never work for this PR as it's running the build workflow from main - rather than the branch.

(One of the many limitations of Github Actions is that you can't trigger a workflow run on a PR that creates the workflow - it has to be merged into main first 😞)

[sbe-arg]

Can we test this in a fork and show proof then we merge master against master.
Is not urgent? Ill try to test during the weekend.

[sbe-arg]

But at first sight I like it.

[sammcj]

@sbe-arg note I believe the test-action is only failing to pull the image because - it doesn't exist yet 😄

[sbe-arg]

Merge and ping me if anything goes wrong but looks good

[sammcj]

Thanks, will do and I'll just revert it right away, no point in stuffing around etc... 😄

[sammcj]

aww 😢 looking into why the post-merge build failed

[sammcj]

follow up PR up #219
if that doesn't work, I'll revert.