Draft: Resolve "Add ctrl-agent to the image(s)"

Closes #3 (closed).

Moving dockerfiles to dedicated directories was necessary so that they can be referenced from docker-compose.yaml. If there's another way, I'm open to it.

This adds a network_mode: host. Without it, I am not able to send DHCP traffic to Kea, nor commands to kea-ctrl-agent, no matter what ports I expose. It might require kea-dhcp4 & kea-ctrl-agent to be able to bind to all addresses if we want them to operate outside network mode host. We might get away with that on kea-dhcp4 with some configuration in interfaces-config, but it doesn't seem possible for kea-ctrl-agent. network_mode: host is a feature that is only available for Linux. That's the minor inconvenience, if there is one. I suggest we leave it like this, and maybe investigate separately. Using this feature also makes exposing the ports redundant, but I would leave those properties there as well.