[CI] Push images to GitHub Container registry too

[eitsupi]

Close #371

Add tags like ghcr.io/rocker-org/r-ver:latest to push the same images to GitHub Container registry as to DockerHub.
Of course, the GitHub Actions workflow files also need a little updating to push images to ghcr.

[eitsupi]

@cboettig For now, no new tags have been added to images prior to R 4.1.3. How do you think it?

[cboettig]

Looks reasonable to me. We might want to push the older tags as well for completeness but probably not a priority. Does this need any further config on my end for the GH registry?

[eitsupi]

No secret or other settings are required, but additional steps would have been needed to make the images registered in the registry public (should be after the package is first registered...).
At any rate, we should see it after the next build is done.

[eitsupi]

I triggered the extra build workflow by hand (to build dev-osgeo) and it works!
https://github.com/rocker-org/rocker-versioned2/pkgs/container/geospatial

[eitsupi]

In the end, no additional steps were needed.
I simply added a step to log into GHCR on the workflow and added the tags and it worked.
(I did change the settings to make images registered in this repository invisible a long time ago because it was confusing, but that has nothing to do with the tags that now pushes).

[eitsupi]

We can verify that images with the exact same digest are available on DockerHub and GitHub.

[cboettig]

this is fantastic. Excellent work, thanks @eitsupi !