build: recommend Xcode 13 or later

Change-Id: Iff44fe2b1b0f8156660f173e9bf6a4399303c75f
diff --git a/Dockerfile b/Dockerfile
index bf5965c..99bcb88 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -42,7 +42,8 @@
 COPY --link --from=build /usr/bin/ndn-dissect /usr/bin/
 
 RUN --mount=from=build,source=/deps,target=/deps \
-    apt-get install -Uy --no-install-recommends $(cat /deps/ndn*) \
+    apt-get install -Uy --no-install-recommends \
+        $(cat /deps/ndn*) \
     && apt-get distclean
 
 ENV HOME=/config