A docker container with every version of the Java JDK 6 to 16 installed in it, and maven to work with them all, for building and testing java applications across all versions.
Without arguments it will execute `.ci/build.sh` once for each version of Java installed, setting the env variables JAVA_VERSION (a number), JAVA_HOME, M2_HOME, and PATH appropriately so invocations of `mvn` and `java`*just work*. If you want to call another script each time: