Add `--cpus` support for `docker update`

This fix tries to address the issue raised in 31032 where it was
not possible to specify `--cpus` for `docker update`.

This fix adds `--cpus` support for `docker update`. In case both
`--cpus` and `--cpu-period/--cpu-quota` have been specified,
an error will be returned.

Related docs has been updated.

Integration tests have been added.

This fix fixes 31032.

This fix is related to 27921, 27958.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

command/container/update.go

@@ -28,6 +28,7 @@ type updateOptions struct {
 	memorySwap         opts.MemSwapBytes
 	kernelMemory       opts.MemBytes
 	restartPolicy      string
+	cpus               opts.NanoCPUs
 
 	nFlag int
 
@@ -66,6 +67,9 @@ func NewUpdateCommand(dockerCli *command.DockerCli) *cobra.Command {
 	flags.Var(&opts.kernelMemory, "kernel-memory", "Kernel memory limit")
 	flags.StringVar(&opts.restartPolicy, "restart", "", "Restart policy to apply when a container exits")
 
+	flags.Var(&opts.cpus, "cpus", "Number of CPUs")
+	flags.SetAnnotation("cpus", "version", []string{"1.29"})
+
 	return cmd
 }
 
@@ -97,6 +101,7 @@ func runUpdate(dockerCli *command.DockerCli, opts *updateOptions) error {
 		CPUQuota:           opts.cpuQuota,
 		CPURealtimePeriod:  opts.cpuRealtimePeriod,
 		CPURealtimeRuntime: opts.cpuRealtimeRuntime,
+		NanoCPUs:           opts.cpus.Value(),
 	}
 
 	updateConfig := containertypes.UpdateConfig{