Shadow the bogus /etc/resolv.conf of centos:8 to fix wksctl

[twelho]

The centos:8 image currently used as the base for weaveworks/ignite-centos:latest includes a bogus /etc/resolv.conf file (that normally gets shadowed by Docker):

$ docker save centos:8 | tar -xf - -O cbab9bd72bc8f39dfc72b687ab56cd464589268d2a468ea8104fee89e4ca8b84/layer.tar | tar xO etc/resolv.conf
# Generated by NetworkManager
nameserver 192.168.122.1

This gets picked up by Ignite as a custom resolver configuration, so it won't be replaced with the symlink to the in-kernel DHCP at /proc/net/pnp. Subsequently, this then breaks wksctl integration as the Footloose-managed VMs don't have proper resolving configured resulting in no internet access (this probably affects Firekube as well).

Fix this by copying a blank file over /etc/resolv.conf during build.

[chanwit]

LGTM