public Optional<String> getDefaultMachine() {
        String ls = CommandLine.runShellCommand(executableName, "ls", "-q");
        List<String> machineNames = Arrays.asList(ls.split("\n"));

        String envMachineName = System.getenv("DOCKER_MACHINE_NAME");

        if (machineNames.contains(envMachineName)) {
            LOGGER.debug("Using docker-machine set in DOCKER_MACHINE_NAME: {}", envMachineName);
            return Optional.of(envMachineName);
        } else if (machineNames.contains("default")) {
            LOGGER.debug("DOCKER_MACHINE_NAME is not set; Using 'default' docker-machine", envMachineName);
            return Optional.of("default");
        } else if (machineNames.size() > 0) {
            LOGGER.debug(
                "DOCKER_MACHINE_NAME is not set and no machine named 'default' found; Using first machine found with `docker-machine ls`: {}",
                machineNames.get(0)
            );
            return Optional.of(machineNames.get(0));
        } else {
            return Optional.empty();
        }
    }
Obtain an instance of the DockerMachineClient wrapper.

@return the singleton instance of DockerMachineClient
core/src/main/java/org/testcontainers/utility/DockerMachineClient.java