[Springboot] Add springboot barebones

[sonagarajan]

Dockerize a starter springboot app and add to stackbox. This will invlove the following steps.

1. Dockerize basic springboot app (without any endpoints) with multistage build
2. Add code folder with Dockerfile to repo
3. Add springboot as an option in stackbox.sh

Refer to Angular and React PRs for more understanding.