Strip illegal symbols from environmental variables in 'postgres:create'

I've come across an issue using your great plugin with a app that contains a ``-``, e.g. ``my-app``. With that, creating a new database fails due to the restriction of characters allowed in Postgres database names. I've updated the ``docker run`` command so that unallowed characters are stripped out. 

I've also set the ``POSTGRES_DB`` variable to the app name. This might be personal preference but it makes more sense to me than having the default ``docker`` database created. I'm happy to remove that if you disagree.

Looking forward to your thoughts about this and thanks for a great plugin.