container: make dockerfile variable officially supported and more modular

roles/wordpress/tasks/main.yml

@@ -2,7 +2,7 @@
 
 - name: Reset dockerfile variable
   set_fact:
-    dockerfile: {}
+    dockerfile: []
 
 - name: Set docker service full name (required because docker_env uses it)
   set_fact:
@@ -11,14 +11,16 @@
 
 - name: Add memcached php extension to container
   set_fact:
-    dockerfile: "{{ dockerfile | combine({'run': \
-      ['apt-get update && apt-get install -y libmemcached-dev libssl-dev zlib1g-dev && \
-      pecl install memcached-3.2.0 && docker-php-ext-enable memcached']}, list_merge='append') }}"
+    dockerfile: >
+      "{{ dockerfile +
+      ['RUN apt-get update && \
+      apt-get install -y libmemcached-dev libssl-dev zlib1g-dev && \
+      pecl install memcached-3.2.0 && docker-php-ext-enable memcached'] }}"
   when: "'memcached' in docker_additional_services"
 
 - name: Add pdo_mysql php extension to container
   set_fact:
-    dockerfile: "{{ dockerfile | combine({'run': ['docker-php-ext-install -j$(nproc) pdo_mysql']}, list_merge='append') }}"
+    dockerfile: "{{ dockerfile + ['RUN docker-php-ext-install -j$(nproc) pdo_mysql'] }}"
   when: "'pdo_mysql' in wordpress_php_extensions"
 
 - name: Include additional volume vars