build: recommend Xcode 13 or later

Change-Id: Ic2c323adade7947326ed7a6cf40a4525ad084987
diff --git a/Dockerfile b/Dockerfile
index 9d9de95..92bc364 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -54,7 +54,8 @@
 COPY --link --from=build /etc/ndn/nlsr.conf.sample /config/nlsr.conf
 
 RUN --mount=from=build,source=/deps,target=/deps \
-    apt-get install -Uy --no-install-recommends $(cat /deps/nlsr) \
+    apt-get install -Uy --no-install-recommends \
+        $(cat /deps/nlsr) \
     && apt-get distclean
 
 ENV HOME=/config