build+docs: recommend Xcode 13 or later

We are not testing Xcode 12 / macOS 11 in CI anymore

Change-Id: I14f2800caab6cd105cf47714d2daab831d2ea587
diff --git a/Dockerfile b/Dockerfile
index c5833b6..5ade25a 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -22,30 +22,31 @@
 ARG JOBS
 ARG SOURCE_DATE_EPOCH
 RUN --mount=rw,target=/src <<EOF
-set -eux
-cd /src
-./waf configure \
-    --prefix=/usr \
-    --libdir=/usr/lib \
-    --sysconfdir=/etc \
-    --localstatedir=/var \
-    --sharedstatedir=/var \
-    --disable-static \
-    --enable-shared
-./waf build
-./waf install
-mkdir -p /deps/debian
-touch /deps/debian/control
-cd /deps
-dpkg-shlibdeps --ignore-missing-info /usr/lib/libndn-cxx.so.* /usr/bin/ndnsec -O \
-    | sed -n 's|^shlibs:Depends=||p' | sed 's| ([^)]*),\?||g' > ndn-cxx
+    set -eux
+    cd /src
+    ./waf configure \
+        --prefix=/usr \
+        --libdir=/usr/lib \
+        --sysconfdir=/etc \
+        --localstatedir=/var \
+        --sharedstatedir=/var \
+        --disable-static \
+        --enable-shared
+    ./waf build
+    ./waf install
+    mkdir -p /deps/debian
+    touch /deps/debian/control
+    cd /deps
+    dpkg-shlibdeps --ignore-missing-info /usr/lib/libndn-cxx.so.* /usr/bin/ndnsec -O \
+        | sed -n 's|^shlibs:Depends=||p' | sed 's| ([^)]*),\?||g' > ndn-cxx
 EOF
 
 
 FROM ubuntu:24.04 AS runtime
 
 RUN --mount=from=build,source=/deps,target=/deps \
-    apt-get install -Uy --no-install-recommends $(cat /deps/ndn-cxx) \
+    apt-get install -Uy --no-install-recommends \
+        $(cat /deps/ndn-cxx) \
     && apt-get distclean
 
 RUN --mount=from=build,source=/usr,target=/build \