Adapt devcontainer.json for systemd-based devcontainer image

Update devcontainer.json settings to work with the new systemd-based
devcontainer image (v4).

- Bump image tag to 4-supervisor to get the systemd-enabled image
- Set overrideCommand to false so the image's CMD (/sbin/init) runs
  as PID 1 instead of being replaced by VS Code's default sleep command
- Set remoteUser to vscode to preserve the non-root shell experience
  (required when overrideCommand is false, since VS Code no longer
  injects its own user-switching wrapper)
- Add /var/lib/containerd volume mount because modern Docker uses the
  containerd snapshotter, which stores layer data outside
  /var/lib/docker
- Add tmpfs on /tmp to match typical systemd expectations and avoid
  leftover state across container restarts

.devcontainer/devcontainer.json

@@ -1,6 +1,8 @@
 {
   "name": "Supervisor dev",
-  "image": "ghcr.io/home-assistant/devcontainer:3-supervisor",
+  "image": "ghcr.io/home-assistant/devcontainer:4-supervisor",
+  "overrideCommand": false,
+  "remoteUser": "vscode",
   "containerEnv": {
     "WORKSPACE_DIRECTORY": "${containerWorkspaceFolder}"
   },
@@ -46,6 +48,8 @@
   },
   "mounts": [
     "type=volume,target=/var/lib/docker",
-    "type=volume,target=/mnt/supervisor"
+    "type=volume,target=/var/lib/containerd",
+    "type=volume,target=/mnt/supervisor",
+    "type=tmpfs,target=/tmp"
   ]
 }