docker-sbx gateway: sbx ssh proxy needs the sbx cache and state paths

[czpython]

The gateway's loopback tunnel (#57) runs sbx ssh proxy inside the gateway container. The container runs with HOME=/, and then the tunnel fails:

error: the sandboxd SSH endpoint is disabled; restart sandboxd after enabling it

The daemon has SSH on: sbx ssh proxy on the host works, and the daemon log says ssh server ready. The container's sbx client fails for two reasons:

1. sbx reads its feature flags from $XDG_CACHE_HOME/sandboxes (unleash-toggles-production.json). With HOME=/ it finds none, so it treats feature.ssh as off and never calls the daemon.
2. sbx ssh proxy dials the daemon socket under $XDG_STATE_HOME/sandboxes/sandboxes/sandboxd/sandboxd.sock. It ignores DOCKER_SANDBOXES_API.

This happens with sbx 0.39.0 and 0.45.1 on Linux. On macOS, sbx keeps its cache in ~/Library/Caches/com.docker.sandboxes and ignores XDG_CACHE_HOME, so macOS does not show the failure.

It works when the gateway gets:

environment:
  XDG_CACHE_HOME: ${DRUKS_SBX_HOME}/.cache
  XDG_STATE_HOME: ${DRUKS_SBX_HOME}/.local/state
volumes:
  - ${DRUKS_SBX_HOME}/.cache/sandboxes:${DRUKS_SBX_HOME}/.cache/sandboxes
  - ${DRUKS_SBX_HOME}/.local/state/sandboxes/sandboxes/sandboxd/sandboxd.sock:${DRUKS_SBX_HOME}/.local/state/sandboxes/sandboxes/sandboxd/sandboxd.sock

Also: the socket is mounted as a file

The compose file mounts sandboxd.sock as one file. A daemon restart, such as an sbx upgrade or a settings change that needs a restart, makes a new socket. The containers keep the old one and cannot reach the daemon until they are recreated.

Expected

In drukbox (#62):

• diagnose reports when sbx ssh proxy cannot reach the SSH endpoint. It requires the daemon's SSH banner from a probe that selects no sandbox.
• The container recipe in docs/deploy.md mounts the directory of the daemon socket, not the socket file. It sets XDG_CACHE_HOME and XDG_STATE_HOME, and it mounts the sbx cache. The gateway container needs the same mounts and variables.

In the deployment's compose file:

• The drukbox services get the sbx cache and state paths, as the recipe shows.
• The compose file mounts the directory of the daemon socket, not the socket file, so a daemon restart does not cut the containers off.
• On the deploy host, sbx ssh proxy from the gateway container prints an SSH banner, also after a daemon restart without a container restart.