Introduce DOCKER_VOLUMES_PATH_PREFIX for easier storage and backup handling

Pascal Ernsterrequested to merge
pascal/bind-mounts into develop

This feature is intended to allow for persisting the Docker volumes in a custom directory tree instead of below /var/lib/docker/volumes/.

That directory tree is created in the path specified in the DOCKER_VOLUMES_PATH_PREFIX variable in the .env file by executing the create_volume_directories.sh script. If the DOCKER_VOLUMES_PATH_PREFIX variable is empty or unset, Docker's default (usually /var/lib/docker/volumes/, albeit with a slightly different directory structure) is used. The script is designed to be idempotent, meaning executing it on a preexisting directory tree will not delete any data but merely add missing directories and adjust file system permissions/ACLs.

Check the README file for instructions on how to migrate from the Docker default to a custom directory tree, and how to create/restore backups.

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