Use gateway specified on command line "docker network create"

[vladwing]

Currently dwgd doesn't set a default route and when it does set it using dwgd.route option, docker gets confused. Allow setting a default route using docker network create ... --gateway <IP>.

Copying from Issue #4, for context

The problem with the current implementation is that the routes are added as static routes, but docker doesn't understand it is 
dealing with a default route. Because of that, if you add another network to a container - for example a bridge, docker will see 
that the dwgd network doesn't provide a default route and will create a default route via the other network. You'll get an 
annoying message saying the container wasn't able to be created because 0.0.0.0/0 already exists. Currently dwgd doesn't use 
the gateway parameter, even though it is specified in the documentation as the way to create the network.

[leomos]

Two general points:

1. can you modify the entities_test and driver_test to take into account your changes? It should be as easy as adding a gateway: <whatever structure we settle on> here:

   dwgd/entities_test.go

   Line 29 in 0e8de0b

   ifname: "dwgd0",

   and something like IPv4Data: []*network.IPAMData{{ Gateway: fmt.Sprintf(<whatever logic to represent the value we settled on>), }}, here:

   dwgd/driver_test.go

   Line 163 in 0e8de0b

   },

2. is it possible to provide an integration test that highlights what wasn't working before and works now? You said

   Because of that, if you add another network to a container - for example a bridge, docker will see that the dwgd network doesn't provide a default route and will create a default route via the other network.

   Maybe a copy-paste of this one with more than one network would be enough?

And of course thanks a lot for sending this!!!

[leomos]

both here and on the IPv6 handling below I would fail if there is an error and return the error, to keep consistency on how it is done in the function.

[vladwing]

Give me a bit of time to update the PR with the requested changes.

[vladwing]

@leomos I would like to update this to a string in a future PR. Thoughts?

[leomos]

it's a good idea to keep consistency with what we agreed over the gateway{_v6} fields.
If you can change it in this PR it's great, otherwise no problem in doing that later.

[vladwing]

What I have in mind is a slightly bigger change. Right now the daemon saves an IP address, which is not even the IP address at the moment of the tunnel creation. I think this should be an FQDN which is resolved at tunnel creation. It will still not solve all the problems the wg-quick solves, but it's a step in that direction and we can iterate over it. We can start by changing the endpoint to a string in this patch and continue with the complicated stuff in a later PR.