code: public DockerRuleMountToBuilder mountFrom(File hostFileOrDir) throws InvalidVolumeFrom {
          if ( ! hostFileOrDir.exists()) {
              throw new InvalidVolumeFrom(String.format("mountFrom: %s does not exist", hostFileOrDir.getAbsolutePath()));
          }
          String hostDirUnixPath = DockerRuleMountBuilder.toUnixStylePath(hostFileOrDir.getAbsolutePath());
          return new DockerRuleMountBuilder(this, hostDirUnixPath);
      }
docs: Host directory to be mounted into container.<br/>
  Please note that in boot2docker environments (OSX or Windows)
  only locations inside $HOME can work (/Users or /c/Users respectively).
  
  @param hostFileOrDir Directory or file to be mounted.