Extend docker_container parent rule to accept vm and proxmox nodes
in addition to docker_host and lxc, covering nested virtualization
topologies (Docker on a VM, Docker directly on Proxmox host).
ha-relevant: yes
Previously docker_container could only attach to a docker_host via
virtual edge. Extend the rule so an LXC node can also act as parent,
enabling docker_container nesting under LXC without requiring an
intermediate docker_host.
Extracts the virtual-edge parent rule into a pure helper
(resolveVirtualEdgeParent) with unit tests covering all type pairs.
ha-relevant: yes
Pouzor