Change mods volume to /home/steam/ror2ds-mods

• The latest version of RoR2DS requires write access to ${STEAMAPPDIR}/Mods/enable_mods.txt, which can halt the server if the mods volume, which is ${STEAMAPPDIR}/mods, is mounted on a read-only directory. Moving the mods volume out of the game data directory may be a better practice.
• Also another minor improvement: When the docker container is reused (e.g. using docker-compose to create container but not docker run), the mods directory ${STEAMAPPDIR}/BepInEx should be cleaned first, to prevent deleted plugins from taking effect again.

Thanks to @mchobbylong for this release!