rootless: add Requires=dbus.socket

On a cgroup v2 host with systemd, dbus is needed to avoid the following error:
```
docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed
: unable to start container process: unable to apply cgroup configuration: unable to start unit "docker-170a4183e351e69835b82cc3134b97c8cbb0e6d3a6
16d5a0fb0ea473075062ad.scope" (properties [{Name:Description Value:"libcontainer container 170a4183e351e69835b82cc3134b97c8cbb0e6d3a616d5a0fb0ea47
3075062ad"} {Name:Slice Value:"user.slice"} {Name:Delegate Value:true} {Name:PIDs Value:@au [2872]} {Name:MemoryAccounting Value:true} {Name:CPUAc
counting Value:true} {Name:IOAccounting Value:true} {Name:TasksAccounting Value:true} {Name:DefaultDependencies Value:false}]): Interactive authen
tication required.: unknown.
```

Signed-off-by: Akihiro Suda <[email protected]>

contrib/dockerd-rootless-setuptool.sh

@@ -314,6 +314,7 @@ install_systemd() {
 			[Unit]
 			Description=Docker Application Container Engine (Rootless)
 			Documentation=https://docs.docker.com/go/rootless/
+			Requires=dbus.socket
 
 			[Service]
 			Environment=PATH=$BIN:/sbin:/usr/sbin:$PATH