Split the container image in a separate repository

[almet]

Utilities to generate the container image, that pushes to the container registry, documentation, security.md.

[apyrgio]

Having the Dangerzone image in a separate repository will allow power users to enhance Dangerzone in interesting ways. See for example #1314, where the end result would not be pixels, but a JSON dict with HTTP links. We could add such an example in the image repo, once we do the separation.