Hey @kwill,
We’ve spoken about this as a team and one option you could try is to include the binary of your desired JVM version and include that in your repo. From there, you could reference the path to that binary to ensure you’re getting the right version for your build tool to run, while eliminating the necessity to install it from scratch every time.
That said, the ability to specify a particular Java version is an excellent idea – if you file a feature request on our build image GitHub repo, we might be able to include this in future versions of our build image 
Let us know if you have any other questions and we’d be happy to help!