@Input
  @Nullable
  @Optional
  public String getExecutable() {
    if (System.getProperty(PropertyNames.DOCKER_CLIENT_EXECUTABLE) != null) {
      return System.getProperty(PropertyNames.DOCKER_CLIENT_EXECUTABLE);
    }
    return executable == null ? null : executable.toString();
  }
Object that configures the Docker executable and the additional environment variables to use when
executing the executable.
jib-gradle-plugin/src/main/java/com/google/cloud/tools/jib/gradle/DockerClientParameters.java