integration: update getExternalAddress to prefer IPv4
integration: update getExternalAddress to prefer IPv4

Rootlesskit doesn't currently handle IPv6 addresses, causing TestNetworkLoopbackNat
and TestNetworkNat to fail;

    Error starting userland proxy:
    error while calling PortManager.AddPort(): listen tcp: address :::8080: too many colons in address

This patch:

- Updates `getExternalAddress()` to pick IPv4 address if both IPv6 and IPv4 are found
- Update TestNetworkNat to net.JoinHostPort(), so that square brackets are used for
  IPv6 addresses (e.g. `[::]:8080`)

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

moul/docker,moul/docker,jameszhan/docker,gnailuy/docker,docker/docker,yosifkit/docker,bearstech/moby,docker/docker,nawawi/docker,yosifkit/docker,djs55/docker,Mokolea/docker,mahak/docker,mahak/docker,Mokolea/docker,Mokolea/docker,nalind/docker,AkihiroSuda/docker,trebonian/docker,mahak/docker,jimjag/docker,jimjag/docker,trebonian/docker,Ramzec/docker,kojiromike/docker,cpuguy83/docker,yosifkit/docker,konstruktoid/docker-upstream,nalind/docker,AkihiroSuda/docker,trebonian/docker,thaJeztah/docker,thaJeztah/docker,jameszhan/docker,thaJeztah/docker,flavio/docker,docker/docker,kojiromike/docker,vdemeester/docker,jameszhan/docker,nawawi/docker,paralin/docker,moby/moby,gnailuy/docker,cpuguy83/docker,gnailuy/docker,Ramzec/docker,flavio/docker,vdemeester/docker,Mokolea/docker,Ramzec/docker,paralin/docker,paralin/docker,mahak/docker,nalind/docker,flavio/docker,nawawi/docker,cpuguy83/docker,bearstech/moby,nawawi/docker,jimjag/docker,konstruktoid/docker-upstream,vdemeester/docker,jthelin/docker,paralin/docker,moby/moby,nalind/docker,thaJeztah/docker,bearstech/moby,moby/moby,moby/moby,Ramzec/docker,AkihiroSuda/docker,jthelin/docker,trebonian/docker,konstruktoid/docker-upstream,kojiromike/docker,cpuguy83/docker,djs55/docker,moul/docker,moul/docker,djs55/docker,gnailuy/docker,jimjag/docker,konstruktoid/docker-upstream,yosifkit/docker,AkihiroSuda/docker,flavio/docker,bearstech/moby