fix(cli): respect rootless Docker in compose socket and SSH host

[santhiprakash]

Problem

openship up --compose hardcoded the rootful Docker socket path and the host.docker.internal SSH host, which breaks rootless Docker installs:

• /var/run/docker.sock:/var/run/docker.sock is not the right socket for rootless Docker.
• extra_hosts: ["host.docker.internal:host-gateway"] and OPENSHIP_HOST_SSH_HOST=host.docker.internal cannot be overridden, so the container cannot reach sshd under rootless networking.

Closes #482.

Changes

• Derive the host-side Docker socket from OPENSHIP_DOCKER_SOCKET, DOCKER_HOST, the active docker context, or rootless fallbacks ($XDG_RUNTIME_DIR/docker.sock, /run/user/<uid>/docker.sock), defaulting to /var/run/docker.sock.
• Always mount the resolved socket at /var/run/docker.sock inside the API container so the API's default still works.
• Make OPENSHIP_HOST_SSH_HOST overridable and preserved across openship up re-runs, defaulting to host.docker.internal.
• Use variable interpolation in the generated compose file so extra_hosts and the socket mount follow the resolved .env values.
• Add unit tests for DOCKER_HOST, XDG_RUNTIME_DIR, explicit .env overrides, and OPENSHIP_HOST_SSH_HOST preservation.

Verification

• bun run --cwd apps/cli lint passes.
• bun run --cwd apps/cli test passes (233 tests).