Add devcontainer postCreateCommand

Author: mikechu-optimizely

.devcontainer/devcontainer.json

@@ -24,7 +24,7 @@
 	],
 
 	// Use 'postCreateCommand' to run commands after the container is created.
-	// "postCreateCommand": "uname -a",
+	"postCreateCommand": "npm install",
 
 	// Set `remoteUser` to `root` to connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
 	"remoteUser": "node"