systemd-resolved has a new behaviour in bionic, in that if there is no
/etc/resolv.conf file when it installs, it assumes it is a fresh
system and makes /etc/resolf.conf a symlink into its compatability
files.
dib ends up saving & restoring whatever /etc/resolv.conf we have after
the inital chroot creation, which may not be what we want -- in the
above case it restores the system-resolved symlink. For
openstack-infra, we use unbound and want simply "127.0.0.1" in a
/etc/resolv.conf file [1].
Formalise the ability to save specific contents into the final image.
Add documentation, and a note in the code that it's an external
interface.
I would have preferred to namespace the .ORIG file with DIB_ or
similar, but this unofficial interface has already escaped into the
wild. Leave it as is for simplicity.
[1] Note that systemd-resolved will obey /etc/resolv.conf as you would
expect, if file exists.
Change-Id: Ie0e97d8072e2b21a54b053fa6fb07b62960c686d
e443700b5c