fix skype bridge being rebuilt when upstream changed

roles/matrix-bridge-mx-puppet-skype/tasks/setup_install.yml

@@ -42,6 +42,7 @@
   docker_image:
     name: "{{ matrix_mx_puppet_skype_docker_image }}"
     source: build
+    force_source: yes
     build:
       dockerfile: Dockerfile
       path: "{{ matrix_mx_puppet_skype_docker_src_files_path }}"