add docker support

[Alex Garel]

https://bugs.launchpad.net/bugs/1717648 reports the lack of support of docker as a bug. It is replied asking to deal with it in before.rules which is fine.

That said, the goal of ufw being to help users dealing with firewall in a simple way, and as docker is very popular, it would be cool to support it.

I see this as ufw interacting with the DOCKER-USER chain in the way it does now with INPUT. The functionality could be switch on/off by configuration (even if having rule in DOCKER-USER without docker, does not harm). There would be a default policy on docker (deny by default), and you could add rules. I imagine it by adding a docker tag to actual [incoming|outgoing|routed].

This is for an upcoming release of ufw.

[Jamie Strandboge (jdstrand)]

It might be possible to do this, but ufw doesn't currently integrate with other software in the manner. At a minimum, we could add information to the man page for this though.

[Jelle Besseling (jellebigbridge)]

> It might be possible to do this, but ufw doesn't currently integrate with other software in the manner.

I think to keep firewalls uncomplicated it would suit ufw well to integrate with docker. Currently it's quite complicated to set correct firewall rules when docker is running. It's even harder to combine this with ufw.

Ufw should help fix this problem by adding docker functionality