public static String firstRegistryOf(String ... checkFirst) {
        for (String registry : checkFirst) {
            if (registry != null) {
                return registry;
            }
        }
        // Check environment as last resort
        return getEnv("DOCKER_REGISTRY");
    }
Return the first non null registry given. Use the env var DOCKER_REGISTRY as final fallback
@param checkFirst list of registries to check
@return registry found or null if none.
src/main/java/io/fabric8/maven/docker/util/EnvUtil.java