For those interested.
The jboss images are created based on centos 7 but there is no openjdk-11 package available there. So I've created this image based on Ubuntu Bionic (18.04) and used the openjdk-11 package that is available there.
Currently the openjdk-11 package on Ubuntu Bionic actually provides openjdk-10. But it should be upgraded in the future.
I've created a tagged image gadeynebram/wildfly14-ubuntu18:real_java_11 where a real openjdk-11 is downloaded from download.java.net.