Add nginx to docker base image

[glensc]

This drops separate nginx container, and therefore avoids the need to synchronize data with fpm and nginx container with webroot volume.

The webroot volume is a source of annoyance when its content changes.
need to prune the volume to catch up updates from it.

For the end-users, they needed to stop containers, remove xhgui_webroot-share docker volume manually.

Note: On upgrade need to delete old container:

$ docker-compose down --rm-orphans

• Cleanup: www-data group is provided by nginx package
• Add volume for nginx pid
• Launch nginx before fpm
• Link nginx logs to stdout/stderr

Note: This switches php-fpm user to www-data (was mistakenly nobody).

/var/www/xhgui # ps axwuf
PID   USER     TIME  COMMAND
    1 root      0:00 php-fpm: master process (/etc/php7/php-fpm.conf)
    8 root      0:00 nginx: master process nginx
    9 nginx     0:00 nginx: worker process
   10 nginx     0:00 nginx: worker process
   11 nginx     0:00 nginx: worker process
   12 nginx     0:00 nginx: worker process
   13 www-data  0:01 php-fpm: pool www
   14 www-data  0:00 php-fpm: pool www
/var/www/xhgui #