DockerHub Optimisation & Extra Build Assets

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GitHub registry should hold images with cache layers, while the docker hub should have the smallest images possible.

• add <code class="notranslate">--squash` (stop using the build action if it can't do this) #6
• run through docker slim, remove everything, the OS only needs enough to build #7
• add container image exports to package repository for offline/air-gapped use #8
• Container run mode, mount local dir, build and spit out artefacts #9