docker_container: show warnings, fix/improve tests (#53440)

* Output warnings from docker daemon on container create and update.

* Accept warning for blkio_weight instead of idempotency.

* Value quoting.

* Avoid loop variable conflict.

* Add changelog.

* Make one test case faster.

* Add 'Docker warning: ' prefix.

* Add a generalized warning reporting function.

lib/ansible/module_utils/docker/common.py

@@ -23,6 +23,7 @@ from distutils.version import LooseVersion
 
 
 from ansible.module_utils.basic import AnsibleModule, env_fallback
+from ansible.module_utils.common._collections_compat import Mapping, Sequence
 from ansible.module_utils.six.moves.urllib.parse import urlparse
 from ansible.module_utils.parsing.convert_bool import BOOLEANS_TRUE, BOOLEANS_FALSE
 
@@ -660,6 +661,20 @@ class AnsibleDockerClient(Client):
 
         return new_tag, old_tag == new_tag
 
+    def report_warnings(self, result, warnings_key=None):
+        '''
+        Checks result of client operation for warnings, and if present, outputs them.
+        '''
+        if warnings_key is None:
+            warnings_key = ['Warnings']
+        for key in warnings_key:
+            if not isinstance(result, Mapping):
+                return
+            result = result.get(key)
+        if isinstance(result, Sequence):
+            for warning in result:
+                self.module.warn('Docker warning: {0}'.format(warning))
+
 
 def compare_dict_allow_more_present(av, bv):
     '''