Gateway network selection ignores "--internal" and "--ipv6" #1141
Description
I've noticed that when connected to multiple (user-defined) networks, NAT and gateway are always configured for the network with the (alphabetically) first network name. This seems a bit random, but I guess it works.
However, in this selection process, internal networks (created with --internal) are not excluded from this list. This means if you're connecting a container to an internal network, internet connectivity for that container will be lost, if the internal network happens to come before other (non-internal) networks it's connected to.
Here's an example showing the issue:
docker run --rm --name=test alpine ping google.com
# ping keeps going (container is connected to network "bridge")
docker network create --internal aaa
docker network create --internal zzz
docker network connect zzz test
# after this, the ping still keeps on going ("zzz" comes after "bridge")
docker network disconnect zzz test
docker network connect aaa test
# the ping now stops! ("aaa" comes before "bridge")
docker network disconnect aaa test
# ping works again
Seems to me this behaviour should not depend on what happens to be the name of the internal network in relation to the names of the other connected networks.
A possible solution would be to only consider non-internal networks for becoming "primary", i.e. select the alphabetically first non-internal network and use it for NAT/gateway. For a container only connected to an internal network, this means no gateway will be configured at all, but this makes sense because why configure a gateway if it won't work.
Similarly, IPv6 should have a separate selection, where it selects the first non-internal network that has IPv6 enabled. Currently, this breaks as well:
docker network create --ipv6 --subnet=fd00:aaa::/48 aaa
docker network create bbb
docker network create --ipv6 --subnet=fd00:ccc::/48 ccc
# a container connected to "aaa" and "bbb" has an IPv6 default gateway
# a container connected to "bbb" and "ccc" does not, even though "aaa" and "ccc" are the same