Apparently this was never implemented.
./src/main/java/jdk/packager/builders/linux/LinuxAppImageBuilder.java://FIXME copyIcon(resourcesDirectory);
// Copy icon to Resources folder
//FIXME copyIcon(resourcesDirectory);
./src/main/java/jdk/packager/builders/linux/LinuxAppImageBuilder.java://FIXME copyIcon(resourcesDirectory);
// Copy icon to Resources folder
//FIXME copyIcon(resourcesDirectory);