feat(devcontainer): add volume mount for root user

Caches VS Code extensions and other profile settings between zmk containers.

Based on advice from KemoNine.

PR: #479

.devcontainer/devcontainer.json

@@ -3,7 +3,10 @@
   "dockerFile": "Dockerfile",
   "runArgs": ["--security-opt", "label=disable"],
   "containerEnv": { "WORKSPACE_DIR": "${containerWorkspaceFolder}" },
-  "mounts": ["type=volume,source=zmk-config,target=/workspaces/zmk-config"],
+  "mounts": [
+    "type=volume,source=zmk-root-user,target=/root",
+    "type=volume,source=zmk-config,target=/workspaces/zmk-config"
+  ],
   "extensions": ["ms-vscode.cpptools"],
   "settings": {
     "terminal.integrated.shell.linux": "/bin/bash"