Use string for postCreateCommand

Author: Chuxel

containers/docker-existing-dockerfile/.devcontainer/devcontainer.json

@@ -15,7 +15,7 @@
 	// "appPort": [],
 
 	// Uncomment the next line to run commands after the container is created.
-	// "postCreateCommand": ["uname -a"],
+	// "postCreateCommand": "uname -a",
 
 	// The optional 'runArgs' property can be used to specify additional runtime arguments.
 	"runArgs": [