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

[dagardner-nv]

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:

chown -R piston:piston ${PISTON_DATA_DIRECTORY:-"/piston"} && \

[SagarBawanthade]

hey i want to work on this issue please assign this issue to me