private void addCopyEntries(StringBuilder b, CopyEntry entry, String topLevelDir, String ownerAndGroup) {
        String dest = topLevelDir + targetDirForEntry(entry) + "/" + entry.destination;
        if (ownerAndGroup == null) {
            DockerFileKeyword.COPY.addTo(b, entry.source, dest);
        } else {
            DockerFileKeyword.COPY.addTo(b, "--chown=" + ownerAndGroup, entry.source, dest);
        }
    }
Create a Dockerfile following the format described in the
<a href="http://docs.docker.io/reference/builder/#usage">Docker reference manual</a>

@return the dockerfile create
@throws IllegalArgumentException if no src/dest entries have been added
src/main/java/io/fabric8/maven/docker/assembly/DockerFileBuilder.java