How would you back up data stored in a Docker volume?

Run a temporary container that mounts the volume read-only alongside a host bind mount, then use a tool like tar to archive the volume's contents to the host (or directly to a backup target like S3). For databases specifically, prefer an application-level backup (pg_dump, mysqldump) over raw filesystem copies, since that produces a consistent, restorable backup rather than a potentially-corrupt snapshot of files mid-write.