Docker image without built-in webserver or reverse proxy

[CyprusSocialite]

Since Docker is the one and only officially supported deployment method from 12.0, it should be made more flexible and more compliant with containerisation "best practices".

To that end I would propose that there needs to be a Docker image running only Seafile components inside (or perhaps separate images for seahub and the server), allowing users to mix and match them with external, manually configured webservers/reverse proxies.

The existing images that come pre-packaged with Nginx could be made to inherit from these smaller, Seafile-only images, so there will still be a simple "out of the box" deployment option.

[CyprusSocialite]

This issue has been raised and gained support in the past as #200, and there is a pull request #203 that was approved but remains open