code: func SetupMountspecs(job *host.Job, artifacts []*ct.Artifact) {
  	for _, artifact := range artifacts {
  		if artifact.Type != ct.ArtifactTypeFlynn {
  			continue
  		}
  		if len(artifact.Manifest().Rootfs) != 1 {
  			continue
  		}
  		rootfs := artifact.Manifest().Rootfs[0]
  		for _, layer := range rootfs.Layers {
  			if layer.Type != ct.ImageLayerTypeSquashfs {
  				continue
  			}
  			job.Mountspecs = append(job.Mountspecs, &host.Mountspec{
  				Type:   host.MountspecTypeSquashfs,
  				ID:     layer.ID,
  				URL:    artifact.LayerURL(layer),
  				Size:   layer.Length,
  				Hashes: layer.Hashes,
  				Meta:   artifact.Meta,
  			})
  		}
  	}
  }
docs: // SetupMountspecs populates job.Mountspecs using the layers from a list of
  // Flynn image artifacts, expecting each artifact to have a single rootfs entry
  // containing squashfs layers