private void addCopy(StringBuilder b) {
        for (CopyEntry entry : copyEntries) {
            String entryUser = userForEntry(entry);
            if (entryUser != null) {
                String[] userParts = entryUser.split(":");
                if (userParts.length > 2) {
                    DockerFileKeyword.USER.addTo(b, "root");
                }
                addCopyEntries(b, entry, "", (userParts.length > 1 ?
                        userParts[0] + ":" + userParts[1] :
                        userParts[0]));
                if (userParts.length > 2) {
                    DockerFileKeyword.USER.addTo(b, userParts[2]);
                }
            } else {
                addCopyEntries(b, entry, "", null);
            }
        }
    }
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