Fix requirement for using root in docker container to use port 80

[magikmw]

Tiny pull request to remove Docker image's dependency on running apache inside the container as root for access to port 80.

This PR also allows to change the port easily, either by modifying the Dockerfile or providing a PORT env variable in the docker run command.

README.md also updated to reflect this change for seamless start.

[ondras]

Thanks @magikmw !