docs: document Docker user permissions and persistent storage

[camathieu]

Restructures the Docker deployment guide to document user permissions, persistent storage options, and common pitfalls.

Changes

• Persistent storage — document named volumes (recommended) and bind mounts as two clear options, with the plikd.cfg configuration required for both
• User permissions — document that the container runs as plik (UID=1000/GID=1000) and bind-mounted host directories must be writable by that UID
• Config path warning — callout that plikd.cfg paths must reference container paths, not host paths
• Setup verification — upload → restart → confirm cycle to catch misconfigurations early
• Docker Compose — modernized example (named volume, docker compose v2 CLI, removed deprecated version key)
• Image tags — added preview tag, clarified version examples
• Removed "Redeploying with a Custom Image" section (internal workflow, not user-facing)

Closes #585