currently they just launch bash

For the builder images the CMD is set in the s2i module:

https://github.com/jboss-container-images/openjdk/blob/4d5246d70bd8f6ea491b5eb50cd18e9a2c6b5a96/modules/s2i/core/api/module.yaml#L119

since the s2i module is not appropriate for the runtime images we can't use the same solution

I suggest a new module (jre.run again?) with a run: cmd : instruction in it pointing at /opt/jboss/container/java/run/run-java.sh