Remove Dockerfile in favor of image

Per the docs, we can directly specify an image in devcontainer.json:
https://code.visualstudio.com/docs/remote/devcontainerjson-reference

.devcontainer/Dockerfile

@@ -1 +0,0 @@
-FROM zmkfirmware/zephyr-west-action-arm

.devcontainer/devcontainer.json

@@ -1,6 +1,7 @@
 {
   "name": "ZMK Development",
-  "dockerFile": "Dockerfile",
-  "extensions": ["ms-vscode.cpptools"]
+  "image": "zmkfirmware/zephyr-west-action-arm",
+  "extensions": ["ms-vscode.cpptools"],
+  "runArgs": ["--security-opt", "label=disable"]
 }