Choose default-cgroup parent by cgroup driver

It's "/docker" for cgroupfs and "system.slice" for systemd.

Fix #19140

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

docs/reference/commandline/daemon.md

@@ -17,10 +17,10 @@ weight = -1
 
     Options:
       --api-cors-header=""                   Set CORS headers in the remote API
-      --authz-plugin=[]                     Set authorization plugins to load
+      --authz-plugin=[]                      Set authorization plugins to load
       -b, --bridge=""                        Attach containers to a network bridge
       --bip=""                               Specify network bridge IP
-      --cgroup-parent=/docker                Set parent cgroup for all containers
+      --cgroup-parent=                       Set parent cgroup for all containers
       -D, --debug                            Enable debug mode
       --default-gateway=""                   Container default gateway IPv4 address
       --default-gateway-v6=""                Container default gateway IPv6 address
@@ -647,7 +647,8 @@ set like this:
 # Default cgroup parent
 
 The `--cgroup-parent` option allows you to set the default cgroup parent
-to use for containers. If this option is not set, it defaults to `/docker`.
+to use for containers. If this option is not set, it defaults to `/docker` for
+fs cgroup driver and `system.slice` for systemd cgroup driver.
 
 If the cgroup has a leading forward slash (`/`), the cgroup is created
 under the root cgroup, otherwise the cgroup is created under the daemon

man/docker-daemon.8.md

@@ -10,7 +10,7 @@ docker-daemon - Enable daemon mode
 [**--authz-plugin**[=*[]*]]
 [**-b**|**--bridge**[=*BRIDGE*]]
 [**--bip**[=*BIP*]]
-[**--cgroup-parent**[=*/docker*]]
+[**--cgroup-parent**[=*[]*]]
 [**--cluster-store**[=*[]*]]
 [**--cluster-advertise**[=*[]*]]
 [**--cluster-store-opt**[=*map[]*]]
@@ -82,7 +82,7 @@ format.
   Use the provided CIDR notation address for the dynamically created bridge (docker0); Mutually exclusive of \-b
 
 **--cgroup-parent**=""
-  Set parent cgroup for all containers. Default is "/docker".
+  Set parent cgroup for all containers. Default is "/docker" for fs cgroup driver and "system.slice" for systemd cgroup driver.
 
 **--cluster-store**=""
   URL of the distributed storage backend