Ensure that the latest aptly-publisher image is used

Change-Id: Ib0e2a6cbae63cfaa4ee250e26cbd5be8cd469213
diff --git a/aptly/publisher.sls b/aptly/publisher.sls
index 63e7866..d7b10f1 100644
--- a/aptly/publisher.sls
+++ b/aptly/publisher.sls
@@ -21,6 +21,17 @@
 
 {%- elif publisher.source.engine == 'docker' %}
 
+{% set publisherImage = (publisher.source.image|default('tcpcloud/aptly-publisher')).split(':') %}
+{{ publisherImage[0] }}:
+  dockerng.image_present:
+    - name: {{ publisherImage[0] }}
+{%- if publisherImage|length > 1 %}
+      tag: {{ publisherImage[1] }}
+{%- else %}
+      tag: latest
+{%- endif %}
+      force: true
+
 publisher_wrapper:
   file.managed:
     - name: /usr/local/bin/aptly-publisher