Add force option to service update

Currently, there's no way to restart the tasks of a service without
making an actual change to the service. This leads to us giving awkward
workarounds as in
https://github.com/docker/docker.github.io/pull/178/files, where we tell
people to scale a service up and down to restore balance, or make
unnecessary changes to trigger a restart.

This change adds a --force option to "docker service update", which
forces the service to be updated even if no changes require that.

Since rolling update parameters are respected, the user can use
"docker service --force" to do a rolling restart. For example, the
following is supported:

   docker service update --force --update-parallelism 2 \
   --update-delay 5s myservice

Since the default value of --update-parallelism is 1, the default
behavior is to restart the service one task at a time.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

contrib/completion/bash/docker

@@ -2473,6 +2473,7 @@ _docker_service_update() {
 		--constraint
 		--endpoint-mode
 		--env -e
+		--force
 		--group-add
 		--label -l
 		--limit-cpu

contrib/completion/zsh/_docker

@@ -1185,6 +1185,7 @@ __docker_service_subcommand() {
                 "($help)--arg=[Service command args]:arguments: _normal" \
                 "($help)*--container-label-add=[Add or update container labels]:label: " \
                 "($help)*--container-label-rm=[Remove a container label by its key]:label: " \
+                "($help)--force[Force update]" \
                 "($help)*--group-rm=[Remove previously added user groups from the container]:group:_groups" \
                 "($help)--image=[Service image tag]:image:__docker_repositories" \
                 "($help)--rollback[Rollback to previous specification]" \

docs/reference/commandline/service_update.md

@@ -29,6 +29,7 @@ Options:
       --endpoint-mode string             Endpoint mode (vip or dnsrr)
       --env-add value                    Add or update environment variables (default [])
       --env-rm value                     Remove an environment variable (default [])
+      --force                            Force update even if no changes require it
       --group-add value                  Add additional user groups to the container (default [])
       --group-rm value                   Remove previously added user groups from the container (default [])
       --help                             Print usage
@@ -67,6 +68,12 @@ Updates a service as described by the specified parameters. This command has to
 The parameters are the same as [`docker service create`](service_create.md). Please look at the description there
 for further information.
 
+Normally, updating a service will only cause the service's tasks to be replaced with new ones if a change to the
+service requires recreating the tasks for it to take effect. For example, only changing the
+`--update-parallelism` setting will not recreate the tasks, because the individual tasks are not affected by this
+setting. However, the `--force` flag will cause the tasks to be recreated anyway. This can be used to perform a
+rolling restart without any changes to the service parameters.
+
 ## Examples
 
 ### Update a service
@@ -75,6 +82,19 @@ for further information.
 $ docker service update --limit-cpu 2 redis
 ```
 
+### Perform a rolling restart with no parameter changes
+
+```bash
+$ docker service update --force --update-parallelism 1 --update-delay 30s redis
+```
+
+In this example, the `--force` flag causes the service's tasks to be shut down
+and replaced with new ones even though none of the other parameters would
+normally cause that to happen. The `--update-parallelism 1` setting ensures
+that only one task is replaced at a time (this is the default behavior). The
+`--update-delay 30s` setting introduces a 30 second delay between tasks, so
+that the rolling restart happens gradually.
+
 ### Adding and removing mounts
 
 Use the `--mount-add` or `--mount-rm` options add or remove a service's bind-mounts