Description of problem:

The agent installer code contains a copy of the default /etc/containers/registries.conf, and tries to replace it wholesale when dealing with mirroring. This has multiple issues.

First, the defaults copied into the installer will become out of date - it will be very surprising for the podman team that when they change that file over time they stop getting updated only for assisted-installer codepaths.

Second, writing this file clashes with the MCO's container runtime config controller and ICSPs (which today, will also try to overwrite that file).

Third, the code incorrectly sets mode 0600 today (see https://github.com/openshift/installer/pull/6745)

The correct thing to do here is to use /etc/containers/registries.conf.d/assisted.conf (note the ".d") - and in that file write only the changes we want. The container runtime should merge things properly.

Version-Release number of selected component (if applicable):
All OCP versions that include https://github.com/openshift/installer/commit/04acaba54e6513071caf321d7a33024f7423fd0a

I'm filing this because this will break newer versions of rpm-ostree today, and I'm probably going to have to do an ugly workaround for this.