allow /etc/hosts /etc/resolv.conf and /etc/hostname to be changed

Fixes #2267
My attempt to fix #2267
cool - then perhaps 

`Note, these files are not saved by `docker commit`, or between steps in a `docker build`, and so will not be preserved in the resulting image.` 

and then something about them also not persisting when a container is restarted - which tbh, I'm still a little puzzled about.