Username regex is overly strict compared to certain common distros

### Description

While certain interpretations of the spec use the same regex (e.g. Alpine Debian), historically `[a-zA-Z\.]` were all valid characters, and I and others have scripts and environments that rely on username patterns like `first.last`. Other projects handle this by introducing an override flag to relax this when booting an image/os version that allows a more relaxed pattern (see https://github.com/atmoz/sftp/issues/109.). I would really like a simple flag to disable this validation so I don't have to mess with the users in the container after booting in. A template level override of the regex could also work, as this validation is very much image dependant.