docker-py: use host-network for nested build of docker-py
docker-py: use host-network for nested build of docker-py

When building this image docker-in-docker, the DNS in the environment
may not be usable for the build-container, causing resolution to fail:

```
02:35:31 W: Failed to fetch http://deb.debian.org/debian/dists/jessie/Release.gpg  Temporary failure resolving 'deb.debian.org'
```

This patch detects if we're building from within a container, and if
so, skips creating a networking namespace for the build by using
`--network=host`.

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

konstruktoid/docker-upstream,flavio/docker,jimjag/docker,docker/docker,trebonian/docker,vdemeester/docker,estesp/docker,ibuildthecloud/docker,cpuguy83/docker,jvanz/docker,AkihiroSuda/docker,jameszhan/docker,jthelin/docker,Ramzec/docker,AkihiroSuda/docker,vdemeester/docker,pradipd/moby,nalind/docker,bearstech/moby,konstruktoid/docker-upstream,estesp/docker,djs55/docker,konstruktoid/docker-upstream,Mokolea/docker,thaJeztah/docker,nawawi/docker,AkihiroSuda/docker,cpuguy83/docker,konstruktoid/docker-upstream,djs55/docker,nawawi/docker,jvanz/docker,pradipd/moby,flavio/docker,ibuildthecloud/docker,unclejack/docker,pradipd/moby,jthelin/docker,yongtang/docker,nawawi/docker,mahak/docker,nalind/docker,jameszhan/docker,cpuguy83/docker,nawawi/docker,dims/docker,kolyshkin/moby,justincormack/docker,thaJeztah/docker,trebonian/docker,thaJeztah/docker,moby/moby,moul/docker,yongtang/docker,mahak/docker,tmp6154/docker,dims/docker,konstruktoid/docker-1,moul/docker,yosifkit/docker,SvenDowideit/docker,flavio/docker,kolyshkin/moby,moby/moby,Ramzec/docker,yosifkit/docker,yosifkit/docker,SvenDowideit/docker,thaJeztah/docker,unclejack/docker,Mokolea/docker,mahak/docker,jimjag/docker,justincormack/docker,moul/docker,gnailuy/docker,yosifkit/docker,bearstech/moby,SvenDowideit/docker,kojiromike/docker,moby/moby,konstruktoid/docker-1,Mokolea/docker,crosbymichael/docker,docker/docker,konstruktoid/docker-1,vdemeester/docker,trebonian/docker,paralin/docker,gnailuy/docker,cpuguy83/docker,justincormack/docker,nalind/docker,yongtang/docker,moby/moby,tmp6154/docker,ibuildthecloud/docker,ibuildthecloud/docker,kolyshkin/moby,paralin/docker,jameszhan/docker,bearstech/moby,nalind/docker,flavio/docker,tmp6154/docker,moul/docker,crosbymichael/docker,gnailuy/docker,gnailuy/docker,Ramzec/docker,crosbymichael/docker,AkihiroSuda/docker,paralin/docker,paralin/docker,jimjag/docker,Mokolea/docker,bearstech/moby,mahak/docker,jvanz/docker,dims/docker,ibuildthecloud/docker,kojiromike/docker,unclejack/docker,Ramzec/docker,djs55/docker,docker/docker,estesp/docker,jimjag/docker,kojiromike/docker,trebonian/docker