docker-outside-of-docker: set moby: false for Debian trixie

mcr.microsoft.com/devcontainers/base:trixie no longer ships moby-cli,
so the docker-outside-of-docker feature errors out at install time.
Disable moby so the feature installs upstream Docker CE instead.

Author: yahonda

.devcontainer/devcontainer.json

@@ -7,7 +7,9 @@
     "ghcr.io/rails/devcontainer/features/ruby:2": {
       "version": "4.0.4"
     },
-    "ghcr.io/devcontainers/features/docker-outside-of-docker:1": {}
+    "ghcr.io/devcontainers/features/docker-outside-of-docker:1": {
+      "moby": false
+    }
   },
   "customizations": {
     "vscode": {