Update devcontainer config - rename remote user and add post-start cmd

Changed the remote user from 'devbox-user' to 'devbox'. Added a post-start command to initialize the Devbox environment and run the Spring Boot application using Maven.

.devcontainer/devcontainer.json

@@ -7,5 +7,6 @@
     "terminal.integrated.shell.linux": "/bin/bash"
   },
   "forwardPorts": [8080],
-  "remoteUser": "devbox-user"
+  "remoteUser": "devbox",
+  "postStartCommand": "bash -c 'source ~/.devbox/shell && mvn spring-boot:run'"
 }