private void addWorkdir(StringBuilder b) {
        if (workdir != null) {
            DockerFileKeyword.WORKDIR.addTo(b, workdir);
        }
    }
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