Add reference to update note.

Config/ProvisionTests/Dockerfiles/arch

@@ -8,6 +8,7 @@ ARG EXTRA_ARCH
 ENV EXTRA_ARCH="$EXTRA_ARCH"
 
 # Arch is rolling, and therefore recommends updating the container immediately.
+#  https://hub.docker.com/_/archlinux#updating
 RUN pacman -Syyu --noconfirm
 
 RUN pacman -Syy  --noconfirm $PACKAGES $EXTRA_ARCH