Vitrage: add missing machine-learning Docker image

A new service was added to Vitrage, the machine learning service.
https://github.com/openstack/vitrage/blob/master/releasenotes/notes/machine_learning_service-da9700e6c6fa61b6.yaml#L2

Change-Id: I78dcb2b57d61483b8996a13b8964b79d7b799cae
Closes-Bug: #1714234

docker/vitrage/vitrage-ml/Dockerfile.j2

@@ -0,0 +1,16 @@
+FROM {{ namespace }}/{{ image_prefix }}vitrage-base:{{ tag }}
+LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build_date }}"
+
+{% block vitrage_ml_header %}{% endblock %}
+
+{% if install_type == 'binary' %}
+
+RUN echo '{{ install_type }} not yet available for {{ base_distro }}' \
+    && /bin/false
+
+{% endif %}
+
+{% block vitrage_ml_footer %}{% endblock %}
+{% block footer %}{% endblock %}
+
+USER vitrage