Dockerfile: update rootlesskit to v1.1.1, and use tags as reference

Commit 0b1c1877c5 updated the version in
hack/dockerfile/install/rootlesskit.installer, but forgot to update the
version in Dockerfile.

Also updating both to use a tag, instead of commit. While it's good to pin by
an immutable reference, I think it's reasonably safe to use the tag, which is
easier to use, and what we do for other binaries, such as runc as well.

Full diff: https://github.com/rootless-containers/rootlesskit/compare/v1.1.0...v1.1.1

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfile/install/rootlesskit.installer

@@ -1,8 +1,7 @@
 #!/bin/sh
 
-# When updating, also update rootlesskit commit in vendor.mod accordingly
-# v1.1.1
-: "${ROOTLESSKIT_VERSION:=a2c596ff9b3fddc0c2becb38f2ef4004f15765b5}"
+# When updating, also update vendor.mod and Dockerfile accordingly.
+: "${ROOTLESSKIT_VERSION:=v1.1.1}"
 
 install_rootlesskit() {
 	case "$1" in