[Docker] Add the ability to select multiple custom networks for containers

[andrebrait]

Disclaimer: this is more of an RFC as I intend to implement this myself and submit a PR.

The problem

Since version 25.0, docker run takes multiple --network arguments, meaning we can now connect containers to multiple (custom) networks. Unraid, however and from what I can see on the master branch here, allows only for a single network to be selected.

The proposal

Turn the network selection field into a checkbox list. Some restrictions must apply, to reflect limitations of the docker run command, such as not being possible to select both bridge/host networks at the same time as custom ones.

Complications

The syntax to expose ports must address the fact ports can now be exposed for different networks.