public synchronized DockerConnectionDetector.ConnectionParameter getConnectionParameter(String certPath) throws IOException {
        if (machine == null) {
            return null;
        }
        if (envMap == null) {
            envMap = getEnvironment();
        }
        String value = envMap.get("DOCKER_HOST");
        if (value == null) {
            return null;
        }
        log.info("DOCKER_HOST from docker-machine \"%s\" : %s", machine.getName(), value);
        return new DockerConnectionDetector.ConnectionParameter(value, certPath != null ? certPath : envMap.get("DOCKER_CERT_PATH"));
    }
launch docker-machine to obtain environment settings
src/main/java/io/fabric8/maven/docker/access/DockerMachine.java