docker_swarm_service: Remove root as default user (#51110)

* Remove root as default user

* Add changelog fragment

* Add deprecation documentation

* Tweak user default docs for docker_swarm_service

Co-Authored-By: hannseman <hannes@5monkeys.se>

* Tweak user default deprecation docs

Co-Authored-By: hannseman <hannes@5monkeys.se>

changelogs/fragments/51110-docker_swarm_service-drop-user-default.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "docker_swarm_service - Don't set ``root`` as the default user."

docs/docsite/rst/porting_guides/porting_guide_2.8.rst

@@ -162,6 +162,8 @@ Noteworthy module changes
 
 * The ``docker_service`` module was renamed to :ref:`docker_compose <docker_compose_module>`.
 
+* The ``docker_swarm_service`` module no longer sets a default for the ``user`` option. Before, the default was ``root``.
+
 Plugins
 =======
 

lib/ansible/modules/cloud/docker/docker_swarm_service.py

@@ -320,12 +320,12 @@ options:
     - Maps to docker service --update-order
     - Requires API version >= 1.29
   user:
+    type: str
     required: false
-    default: root
     description:
-    - username or UID.
-    - "If set to C(null) the image provided value (or the one already
-       set for the service) will be used"
+    - Sets the username or UID used for the specified command.
+    - Before Ansible 2.8, the default value for this option was C(root).
+      The default has been removed so that the user defined in the image is used if no user is specified here.
 extends_documentation_fragment:
 - docker
 requirements:
@@ -558,7 +558,7 @@ class DockerService(DockerBaseClass):
         self.reserve_cpu = 0.000
         self.reserve_memory = 0
         self.mode = "replicated"
-        self.user = "root"
+        self.user = None
         self.mounts = []
         self.configs = []
         self.secrets = []
@@ -1300,7 +1300,7 @@ def main():
         update_monitor=dict(default=5000000000, type='int'),
         update_max_failure_ratio=dict(default=0, type='float'),
         update_order=dict(default=None, type='str'),
-        user=dict(default='root'))
+        user=dict(type='str'))
 
     option_minimal_versions = dict(
         dns=dict(docker_py_version='2.6.0', docker_api_version='1.25'),