Give write access to server creds for pwn3

[videlanicolas]

masterserver.sh writes the new server creds to this folder. This is being called as user pwn3 inside the container and there is no check that the user pwn3 has write access to this file. This makes sure that the permission is granted.

[mattypiper]

This issue occurs for me as well, and I believe approving the pull request will fix it.

For me, I fixed my container with:

docker exec -it $CONTAINER bash
chown -R pwn3:pwn3 /opt/pwn3

I added a sleep 60 to the end of gameserver.sh to keep the container alive after the SIGSEGV so that I could fix the permissions.