This commit replaces all references to `umputun/remark42` Docker images
on Docker Hub with `ghcr.io/umputun/remark42` from the GitHub
Container Registry. It updates various Docker Compose files,
documentation, and the Makefile to use the new image location.
It also updates the kubernetes example to use the latest version.
Docker Hub is going to kill free pulls for too long by now.
After this commit, dev auth would start working with the `REMARK_URL`
hostname instead of the previously hardcoded 127.0.0.1.
Breaks development setup where `REMARK_URL` was set
to a non-standard value and dev auth was running on 127.0.0.1
and working, as, after that change, it would stop working.