Add header blocks to all Dockerfiles

Change needed to add header blocks to all Dockerfiles, similar to the
base.

Use case is to easily run something before packages are installed, e.g.
to COPY a local rpm in that can be added to the package list.

Change-Id: I1bbfdf0b762da0a392aa8bf47781315b45377bee
Closes-Bug: 1618969

docker/designate/designate-mdns/Dockerfile.j2

@@ -1,5 +1,8 @@
 FROM {{ namespace }}/{{ image_prefix }}designate-base:{{ tag }}
 MAINTAINER {{ maintainer }}
+
+{% block designate_mdns_header %}{% endblock %}
+
 {% import "macros.j2" as macros with context %}
 
 {% if install_type == 'binary' %}