add devcontainer example

devcontainer_example.json

+{
+    "name": "Flutter Linux",
+    "image": "andrijoos/flutter:3-linux",
+    "remoteUser": "flutter",
+    "workspaceFolder": "/workdir",
+    "workspaceMount": "source=${localWorkspaceFolder},target=/workdir,type=bind,consistency=delegated",
+    "customizations": {
+        "vscode": {
+            "settings": {
+                "remote.autoForwardPorts": false
+            },
+            "extensions": [
+                "dart-code.dart-code",
+                "dart-code.flutter",
+                "ms-azuretools.vscode-docker"
+            ]
+        }
+    },
+    "containerEnv": {
+        "SHELL": "/bin/bash"
+    },
+    "postCreateCommand": "cd ai_chat && flutter pub get"
+}