This commit adds a Docker Compose file which will run the application
using docker. The application is fronted by Traefik, and runs using
PHP-fpm. nginx is used to accept queries from Traefik and relay them to
the PHP-fpm containers (Traefik doesn't support FastCGI directly).
Sessions are shared between the various PHP containers using memcached.
Part of this work is based on [this repo](https://github.com/wandersonwhcr/docker-nginx-php-fpm)
This initial import is a heavily modified version of the code I had
here, as Arse was modified for other purposes in the meantime and the
application no longer worked with it.
In addition:
* I did not import the user management part yet,
* task dependencies are supported in-base, but there is no interface
for that yet.