[Feat] Keep dockerfiles in the latest release branch up-to-date

[ninsbl]

Is your feature request related to a problem? Please describe.
Currently, development and release-branch build different Docker images, at least for Ubuntu, with different base images and the like. That makes it more difficult to switch between latest release and development version in downstream projects like e.g. actinia.

Describe the solution you'd like
Dockerfiles for the development version and the latest release should be synced as mush as possible. There may e cases where the development branch supports newer versions of dependencies (thonk e.g. GDAL, PROJ, PDAL, ...) that the latest release does not support. The same may apply to changes in the build system. So, different dependency versions should be OK and some adjustments may be necessary but otherwise, changes to dockerfiles should be backported where possible.
Not sure if or how Dockerfiles in the releasebranch can and should be monitored and kept up to date with renovate as well (and if that is in place already)...

Describe alternatives you've considered
Thye alternative would be users building own docker images.

Additional context
Related / relevant PRs and issues are:
#6410
#6481
#6512

[echoix]

More generally, it is possible to have renovate run for more than only the default branch. It's been a while that I'm thinking about how to properly handle this.

Especially for docker base image monthly respins, it makes sense to rebuild our release images with updated security fixes, if any, more often than once a year-1.5 year. Having the beta (main branch) images updated isn't as valuable for users, again if any. Do we know a bit on the usage of these images we build, how many people use it and in what purposes to better align with their needs?

I didn't have the occasion yet to take a look at what can be configured in our renovate.json about multiple branches, to not try to keep all dependencies updated, and not at the same frequency as the main branch.
It would be easier than always backporting.