Use more portable post-install command in devcontainer

Windows breaks 'initializeCommand' by running the command with 'cmd.exe' locally rather than trying to run it in the container. This is apparently what the command is supposed to do which is not what I realized originally, so changed it to postCreateCommand which specifically runs in the container as intended.

Author: derickson2402

devcontainer.json

@@ -24,7 +24,7 @@
 	//     https://github.com/derickson2402/Dockerized-CAEN/issues
 	// In this case, we are installing the tree command (even though it is
 	// there by default, so this is not a useful example...):
-	// "initializeCommand": "dnf update -y && dnf install -y tree",
+	// "postCreateCommand": "dnf update -y && dnf install -y tree",
 
 	// Give us a pretty bash prompt (since VSCode is overriding shell)
 	"remoteEnv": {