Fix comment lines about the default mount propagation

dockerd's default propagation is "rprivate", not "private":
https://github.com/moby/moby/blob/v20.10.23/volume/mounts/linux_parser.go#L145

However, dockerd automatically changes the default propagation to "rslave"
when the mount source contains the daemon root (`/var/lib/docker`):
- https://github.com/moby/moby/blob/v20.10.23/daemon/volumes.go#L137-L143
- https://github.com/moby/moby/blob/v20.10.23/daemon/volumes_linux.go#L11-L36

This behavior was introduced in Docker 18.03 (moby/moby PR 36055).

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

libdocker/helpers.go

@@ -233,14 +233,23 @@ func GenerateMountBindings(mounts []*v1.Mount, terminationMessagePath string) []
  }
  switch m.Propagation {
  case v1.MountPropagation_PROPAGATION_PRIVATE:
- // noop, private is default
+ // noop, dockerd will decide the propagation.
+ //
+ // dockerd's default propagation is "rprivate": https://github.com/moby/moby/blob/v20.10.23/volume/mounts/linux_parser.go#L145
+ //
+ // However, dockerd automatically changes the "private" propagation to "rslave"
+ // when the mount source contains the daemon root (/var/lib/docker):
+ // - https://github.com/moby/moby/blob/v20.10.23/daemon/volumes.go#L137-L143
+ // - https://github.com/moby/moby/blob/v20.10.23/daemon/volumes_linux.go#L11-L36
+ //
+ // This behavior was introduced in Docker 18.03: https://github.com/moby/moby/pull/36055
  case v1.MountPropagation_PROPAGATION_BIDIRECTIONAL:
  attrs = append(attrs, "rshared")
  case v1.MountPropagation_PROPAGATION_HOST_TO_CONTAINER:
  attrs = append(attrs, "rslave")
  default:
  logrus.Infof("Unknown propagation mode for hostPath %s", m.HostPath)
- // Falls back to "private"
+ // let dockerd decide the propagation
  }
 
  if len(attrs) > 0 {