hack/zagreb: Add contents for the third session

Signed-off-by: Stefan Jumarea <[email protected]>

content/hackathons/2024-05-zagreb/index.mdx

@@ -108,4 +108,8 @@ After you are done with all of them, take a look to some more applications, foll
 
 ### Using Docker and Docker-based Filesystems
 
+In order to bring your own application on KraftCloud, you need to build a minimal required filesystem.
+We do that using `docker`, then we will run the application using `kraft`.
+Follow the steps [here](https://unikraft.org/guides/using-the-app-catalog) (only the ones under `Redis`) to see how you can port a new application on top of Unikraft.
+
 ### Debugging Unikraft / KraftCloud Applications