Allow to run the container as non root

[quentinus95]

Following #157, I would like to know if it could be possible to allow using the user Docker directive, instead of only the environment variables.

Current configuration requires running the container as root, and then trust that it will switch to the specified UID/GID. This presents a security risk.

Could it be possible to allow the container to run as a non root? It was possible before version 0.13.

To test it, you can simply use the command:

docker run -it --user 2000 frooodle/s-pdf:0.15.1