public Builder addVolume(AbsoluteUnixPath volume) {
      volumes.add(volume);
      return this;
    }
Adds a directory that may hold an externally mounted volume.

@param volume a directory path on the container filesystem to represent a volume
@return this
@see #setVolumes(Set)
jib-build-plan/src/main/java/com/google/cloud/tools/jib/api/buildplan/ContainerBuildPlan.java