Hello,
Dockerfile are opensource : https://github.com/SonarSource/docker-sonarqube You can see that we currently are FROM openjdk:11 that you can found here : https://github.com/docker-library/openjdk
More generally, you can inspect a docker image with docker image inspect https://docs.docker.com/engine/reference/commandline/image_inspect/
Or use some tools like dive https://github.com/wagoodman/dive
If you want to build an image on top of something else than openjdk:11, feel free to fork the public docker-sonarqube github repo, edit the Dockerfile and build your own image fitting your needs.