`api/src/docker-entrypoint.sh` should use `PISTON_DATA_DIRECTORY` env variable

On line 28 of `api/src/docker-entrypoint.sh` the ownership of the `/piston` directory is changed, I believe this command should use the `PISTON_DATA_DIRECTORY` env variable if defined, defaulting to `/piston` otherwise. Something like: ```sh chown -R piston:piston ${PISTON_DATA_DIRECTORY:-"/piston"} && \ ```