The stub DNS server is authoritative for the RRs we synthesize, such as
localhost, _gateway, and entries from /etc/hosts, and also for trust anchors.
Partially fixes https://github.com/systemd/systemd/issues/17972
Docker doesn't set $container, so it cannot be detected that way. Instead, we
check for presence of /.dockerinit, which it creates. Podman does set
$container, but some Red Hat images (in particular, Fedora images) override
$container to equal "oci". So to correctly detect Podman containers, we check
for presence of /run/.containerenv, which is created by Podman and is now the
official way to get information about the container from within the container.
Fixes https://github.com/systemd/systemd/issues/15393
Sergey Bugaev