Configurable PUBLIC_URL with docker images

[mcserep]

On the https://codecompass.net website we use the /demo/ path prefix to serve a staging instance of the CodeCompass webserver. On the server itself, CodeCompass is executed in Docker containers, where the new frontend is built in the docker images with the PUBLIC_URL environment set to empty string. Therefore all static resources (JS files, images, etc.) are loaded from https://codecompass.net/new/, instead of https://codecompass.net/demo/new/, resulting in 404 errors.
(Currently it only works because some manual fixing was done.)

Since we are talking about Docker images, it would be nice not to hard code the path into it like that. Can we make it a little more dynamic, configurable at runtime?