mkconcore generates Octave Dockerfiles for Java nodes and omits concore.java #634

In a Docker setup, Java nodes currently fail to build because mkconcore.py treats them as Octave nodes instead of Java.

Although Dockerfile.java already exists in the repository, mkconcore.py doesn't handle langext == "java" during Dockerfile selection. As a result, it falls back to Dockerfile.m and also misses the Java CMD instruction.

Additionally, Java runtime files (concoredocker.java / concore.java) are not copied to the required locations, leaving the container without the necessary Java runtime.

This can be reproduced using:

concore build example/java_e2e/java_e2e.graphml --source example/java_e2e --type docker

I'd like to work on fixing these issues and submit a PR to add proper Java support for Docker builds.