Add Dockhand container management service

[xrhstosmour]

What:

1. Adds a self-hosted Dockhand Docker container management UI at management/services/dockhand, following the existing docker-compose.yml + template.env pattern, mounting /var/run/docker.sock and a named data volume.
2. Skips wiring it to the shared postgresql container despite official support for it: reproduced Finsys/dockhand#1110 locally on the latest v1.0.40 image, where Postgres migrations fail with CREATE SCHEMA IF NOT EXISTS "drizzle" against a fresh database, a confirmed and currently unresolved upstream bug. Left a TODO to switch once it's fixed upstream; ships on the image's default embedded SQLite for now.

Why:

Pre-staging Docker management tooling for a future VPS install.

Testing:

1. docker compose config validates with no errors.
2. docker compose up -d starts the container, reaches healthy status.
3. curl localhost:<port>/ returns 200.
4. Reproduced the Postgres-backed failure on a fresh DB before falling back to SQLite.
5. docker compose down -v cleans up with no leftover state.