Add a devcontainer configuration (#4969)

.devcontainer/devcontainer.json

@@ -0,0 +1,17 @@
+{
+  "name": "pallets/flask",
+  "image": "mcr.microsoft.com/devcontainers/python:3",
+  "customizations": {
+    "vscode": {
+      "settings": {
+        "python.defaultInterpreterPath": "${workspaceFolder}/.venv",
+        "python.terminal.activateEnvInCurrentTerminal": true,
+        "python.terminal.launchArgs": [
+          "-X",
+          "dev"
+        ]
+      }
+    }
+  },
+  "onCreateCommand": ".devcontainer/on-create-command.sh"
+}