public String lookup() {
            String user = getProperty(DOCKER_IMAGE_USER);
            if (user != null) {
                return user;
            }
            String groupId = project.getGroupId();
            while (groupId.endsWith(".")) {
                groupId = groupId.substring(0, groupId.length() - 1);
            }
            int idx = groupId.lastIndexOf(".");
            return sanitizeName(groupId.substring(idx != -1 ? idx + 1 : 0));
        }
Property to lookup for image user which overwrites the calculated default (group).
Used with format modifier %g
src/main/java/io/fabric8/maven/docker/util/ImageNameFormatter.java