Option to set cookie domain during install or from flarum.env

[hynsey]

I'd like to set my cookie domain in the config.php file (undocumented, but available properties mentioned in this thread (https://discuss.flarum.org/d/29936-manage-flarum-session/4):

'cookie' => 
  array (
    'path' => '/',
    'domain' => '.flarum.tld',
  )

I'd like to set a wildcard cookie here, so that I can set a wildcard cookie on my other .net Core app which runs on the root of the domain e.g. domain.tld, with Flarum running on flarum.domain.tld

Is there any chance of implementing, or providing guidance on how to do this, so that I don't have to enter my container each time it starts and manually set it, then restart Flarum within the container?

[hynsey]

I've created a fork and implemented this: https://github.com/hynsey/docker-flarum

Please review, would be nice to get this merged if it's sufficient.