Add matrix_docker_installation_enabled

roles/matrix-base/defaults/main.yml

@@ -42,6 +42,10 @@ matrix_docker_network: "matrix"
 # matrix domain (`matrix_server_fqn_matrix`).
 matrix_well_known_matrix_server_enabled: true
 
+# Controls whether docker is automatically installed.
+# If you change this to false you must install and update docker manually.
+matrix_docker_installation_enabled: true
+
 # Variables to Control which parts of our roles run.
 run_postgres_import: true
 run_postgres_upgrade: true