You can find the App Images at System → Apps → Images.
You can either upload an image that you have or search for an image in the Docker Registry. The Docker Registry has a lot of prepared images you can use.
Please visit the Docker Hub website to browse available Images Dockerhub.
If the VT AIR has a working internet connection you can enter a search word and it will show you any docker image that matches the word. You can select it and it will be downloaded for use in a Container.