Use python3 when creating the docker image tags

python2 will be gone away soon so switch to python3.

Change-Id: If0511db6fbd97fe72184b3fa1ef92f29f8cae4c7

tools/gate/playbooks/docker-image-build.yaml

@@ -28,7 +28,7 @@
             msg: "{{ tags | to_json }}"
 
     - name: Determine tags
-      shell: echo '{{ tags | to_json }}' | python {{ zuul.project.src_dir }}/tools/image_tags.py
+      shell: echo '{{ tags | to_json }}' | python3 {{ zuul.project.src_dir }}/tools/image_tags.py
       environment:
         BRANCH: "{{ zuul.branch }}"
         CHANGE: "{{ zuul.change }}"

tools/image_tags.py

@@ -1,4 +1,4 @@
-#!/bin/python
+#!/usr/bin/python3
 # Copyright 2018 AT&T Intellectual Property.  All other rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");