Add support for specifying docker swarm subnet ranges

[tomdaley92]

Ran into an issue IP conflict recently that took me 2 weeks to debug 😨 ... Only to find out that docker swarm creates overlay networks starting with the private class A subnet range 10.0.0.0/8 and goes up from there sequentially, per swarm.

Basically I was having issues with my pfSense OpenVPN connection that I set to subnet 10.0.0.0/24. Everything worked, except I was unable to load web pages served from docker swarm lol. Using docker network inspect ingress I found that swarm was setting the this network to 10.0.0.0/24 and any odditional overlay networks to 10.0.1.0/24, then 10.0.2.0/24, and so on...

Hopefully this helps someone out in the future, but figured I should just configure swarm to start the subnet ranges from something like 10.200.0.0/16 to reduce any conflicts in the future, as the 10.0.0.0/8 block is a popular choice for VLAN subnets.

[tomdaley92]

When I "refresh" my VMs next, this will be a good time to do this, due to the fact that you can only change the subnet ranges during a docker swarm init.

[tomdaley92]

per docs this is acheived by creating the docker network before running docker swarm init

For example the order would be:

sudo apt install docker-ce

docker network create \
--subnet=10.200.0.0/16 \
--gateway 10.200.0.1 \
-o com.docker.network.bridge.enable_icc=false \
-o com.docker.network.bridge.name=docker_gwbridge \
-o com.docker.network.bridge.enable_ip_masquerade=true \
docker_gwbridge

docker swarm init