oci: partially restore comment on read-only mounts for uid/gid uses

Commit cab0562 removed the tryReadonlyMounts
utility, in favor of mounts.ReadOnlyMounts() that was added in commit
daa3a76.

That change made part of the comment redundant, because mounts.ReadOnlyMounts
handles both overlayfs read-only mounts (by skipping the workdir mounts), and
sets the "ro" option for other mount-types, but the reason why we're using a
read-only mount is still relevant, so restoring that part of the comment.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit cb2c3ec)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

oci/spec_opts.go

@@ -683,6 +683,10 @@ func WithUser(userstr string) SpecOpts {
 				return err
 			}
 
+			// Use a read-only mount when trying to get user/group information
+			// from the container's rootfs. Since the option does read operation
+			// only, we append ReadOnly mount option to prevent the Linux kernel
+			// from syncing whole filesystem in umount syscall.
 			return mount.WithReadonlyTempMount(ctx, mounts, f)
 		default:
 			return fmt.Errorf("invalid USER value %s", userstr)
@@ -743,6 +747,10 @@ func WithUserID(uid uint32) SpecOpts {
 			return err
 		}
 
+		// Use a read-only mount when trying to get user/group information
+		// from the container's rootfs. Since the option does read operation
+		// only, we append ReadOnly mount option to prevent the Linux kernel
+		// from syncing whole filesystem in umount syscall.
 		return mount.WithReadonlyTempMount(ctx, mounts, setUser)
 	}
 }
@@ -787,6 +795,10 @@ func WithUsername(username string) SpecOpts {
 				return err
 			}
 
+			// Use a read-only mount when trying to get user/group information
+			// from the container's rootfs. Since the option does read operation
+			// only, we append ReadOnly mount option to prevent the Linux kernel
+			// from syncing whole filesystem in umount syscall.
 			return mount.WithReadonlyTempMount(ctx, mounts, setUser)
 		} else if s.Windows != nil {
 			s.Process.User.Username = username
@@ -865,6 +877,10 @@ func WithAdditionalGIDs(userstr string) SpecOpts {
 			return err
 		}
 
+		// Use a read-only mount when trying to get user/group information
+		// from the container's rootfs. Since the option does read operation
+		// only, we append ReadOnly mount option to prevent the Linux kernel
+		// from syncing whole filesystem in umount syscall.
 		return mount.WithReadonlyTempMount(ctx, mounts, setAdditionalGids)
 	}
 }
@@ -926,6 +942,10 @@ func WithAppendAdditionalGroups(groups ...string) SpecOpts {
 			return err
 		}
 
+		// Use a read-only mount when trying to get user/group information
+		// from the container's rootfs. Since the option does read operation
+		// only, we append ReadOnly mount option to prevent the Linux kernel
+		// from syncing whole filesystem in umount syscall.
 		return mount.WithReadonlyTempMount(ctx, mounts, setAdditionalGids)
 	}
 }