container: support bind mounted db data directory and mysql config

roles/container/meta/argument_specs.yml

@@ -37,6 +37,19 @@ argument_specs:
         description: "database_passwords[docker_service] is a string with the password used for communication between the service and database. Required if docker_database is postgres or mariadb"
         type: dict
         required: false
+      docker_mariadb_config:
+        description: |
+          A dict specifying config for mariadb. For example
+          mysqld:
+            innodb_buffer_pool_size: '4G'
+            aria_pagecache_buffer_size': '4G'
+          would set
+          [mysqld]
+          innodb_buffer_pool_size = 4G
+          aria_pagecache_buffer_size = 4G
+        type: dict
+        required: false
+        default: {}
       docker_additional_services:
         description: "List of additional services to configure (in separate containers). These will be accessible with hostname <docker_service_name>[_suffix]_<additional_service>"
         type: list