Automatically bump our container image parameters

[apyrgio]

It would be nice to have a CI job that bumps the parameters of our Dangerzone image, as outlined in Dockerfile.env. Retrieving the newest values is just a matter of hitting some GitHub/DockerHub APIs (see URLs in Dockerfile.env). We can also create a PR automatically when there's a change, using the gh tool that's installed in the images. Note that this CI job can run once a week, since the Debian base image, gVisor APT repo, and H2ORestart are not updated daily.