buildah bud --dns* flag may conflict with --network

[QiWang19]

Description
Mirror issue from libpod containers/podman#3553
Using --dns* and --network together can have conflicts if using --dns* with
--network 'container:<name|id>'
--network 'none'

Steps to reproduce the issue:

1. buildah build-using-dockerfile and buildah from have --dns* and --network flags. Will this have conflict?

Output of rpm -q buildah or apt list buildah:

buildah-1.9.0-1.git00eb895.fc30.x86_64

Output of buildah version:

$ buildah version
Version:         1.9.3-dev
Go Version:      go1.12.6
Image Spec:      1.0.1
Runtime Spec:    1.0.1-dev
CNI Spec:        0.4.0
libcni Version:  v0.7.0-rc2
Git Commit:      555b5a50
Built:           Mon Jul 22 15:27:19 2019
OS/Arch:         linux/amd64

[mheon]

Background here: we noticed this one while doing some updates to Podman's --network flag. Podman matches Docker's syntax (none, bridge, host, container:$ID). Buildah, however, does not support this syntax - most notable, buildah bud --network container doesn't point to another container, but instead seems to just set up a CNI network for the container.

It's sort of confusing that we have two tools with mostly identical CLI syntax disagreeing completely on this flag.

[rhatdan]

@nalind WDYT?