self-build consistency: add deprecation warnings

roles/matrix-bridge-appservice-slack/tasks/validate_config.yml

@@ -11,3 +11,12 @@
     - "matrix_appservice_slack_homeserver_url"
     - "matrix_appservice_slack_homeserver_token"
     - "matrix_appservice_slack_id_token"
+
+- name: (Deprecation) Catch and report renamed settings
+  fail:
+    msg: >-
+      Your configuration contains a variable, which now has a different name.
+      Please change your configuration to rename the variable (`{{ item.old }}` -> `{{ item.new }}`).
+  when: "item.old in vars"
+  with_items:
+    - {'old': 'matrix_appservice_slack_container_self_build', 'new': 'matrix_appservice_slack_container_image_self_build'}