Docker IPv6 support

Docker's initial IPv6 support is based on allocating public IPv6 to containers.
This approach has many issues:
* The server may not get a block of IPv6 assigned to it
* It's complicated to allocate a block of IPv6 to cloudron server on home setups
* It's unclear how dynamic IPv6 is. If it's dynamic, then should containers be recreated?
* DNS setup is complicated
* Not a issue for Cloudron itself, but with -P, it just exposed the full container into the world

Given these issues, IPv6 NAT is being considered. Even though NAT is not a security mechanism as such,
it does offer benefits that we care about:
* We can allocate some private IPv6 to containers
* Have docker NAT66 the exposed ports
* Works similar to IPv4

Currently, the IPv6 ports are always mapped and exposed. The "Enable IPv6" config option is only whether
to automate AAAA records or not. This way, user can enable it and 'sync' dns and we don't need to
re-create containers etc. There is no inherent benefit is not exposing IPv6 at all everywhere unless we find
it unstable.

Fixes #264

baseimage/initializeBaseUbuntuImage.sh

@@ -89,7 +89,7 @@ echo "==> Installing Docker"
 
 # create systemd drop-in file. if you channge options here, be sure to fixup installer.sh as well
 mkdir -p /etc/systemd/system/docker.service.d
-echo -e "[Service]\nExecStart=\nExecStart=/usr/bin/dockerd -H fd:// --log-driver=journald --exec-opt native.cgroupdriver=cgroupfs --storage-driver=overlay2" > /etc/systemd/system/docker.service.d/cloudron.conf
+echo -e "[Service]\nExecStart=\nExecStart=/usr/bin/dockerd -H fd:// --log-driver=journald --exec-opt native.cgroupdriver=cgroupfs --storage-driver=overlay2 --experimental --ip6tables" > /etc/systemd/system/docker.service.d/cloudron.conf
 
 # there are 3 packages for docker - containerd, CLI and the daemon
 readonly docker_version=20.10.12