common: postinst needs to check for a better valid hostname

[patch0]

The HOSTNAME env variable doesn't tend to bell

It should probably check against a regexp for the presence of a dot in the $HOSTNAME env variable:

/^[_a-z0-9-]+\.([_a-z0-9-]+\.?)+$/

would do the trick

Originally reported on Bytemark's Gitlab by @patch0 on 2017-04-21T19:29:30.820Z

[patch0]

I think this was following testing inside docker containers, where HOSTNAME gets set weirdly, and really shouldn't be used.

[telyn]

it's causing similar problems with systemd containers i think

[telyn]

Is this bug meant to fix-up invalid $HOSTNAMEs by appending '.localdomain', or just complain early in the install process?