Compact Docker server for Postfixadmin

Based on dirtsimple/php-server, this container is a an alpine nginx+php 7.1 runner for Postfixadmin 3.1+. It supports the same configuration variables as hardware/postfixadmin, adding these new ones as well:

• ADMIN_USER -- used to set the admin_email for Postfixadmin (to $ADMIN_USER@$DOMAIN). postfixadmin is the default.
• DEFAULT_ALIASES -- a space-separated list of default mail aliases that will be forwarded from each domain to the same alias at $DOMAIN. Defaults to abuse hostmaster postmaster webmaster.

Performance tuning or custom configuration can be done using any of the environment variables or template configuration capabilities supplied by dirtsimple/php-server. You can also build a specific tag or branch of Postfixadmin by passing it as the POSTFIXADMIN_VERSION build argument. The code/webroot is found under /code.