Fix update.sh Dockerfile status reporting

Compare each generated Dockerfile with its existing copy before
replacing it, so the script only announces an update when the content
actually changes. Otherwise keep the original file and print an
'already up to date' message to eliminate false positives.

Author: PeterDaveHello

update.sh

@@ -165,21 +165,21 @@ function update_node_version() {
       sed -Ei -e "s/(debian:)name-slim/\\1${variant}/" "${dockerfile}-tmp"
     fi
 
-    if diff -q "${dockerfile}-tmp" "${dockerfile}" > /dev/null; then
-      echo "${dockerfile} is already up to date!"
-    else
-      if [ "${SKIP}" != true ]; then
-        sed -Ei -e 's/^(ENV YARN_VERSION)=.*/\1='"${yarnVersion}"'/' "${dockerfile}-tmp"
-      fi
-      echo "${dockerfile} updated!"
+    if [ "${SKIP}" != true ]; then
+      sed -Ei -e 's/^(ENV YARN_VERSION)=.*/\1='"${yarnVersion}"'/' "${dockerfile}-tmp"
     fi
 
     # Required for POSIX sed
     if [ -f "${dockerfile}-tmp-e" ]; then
       rm "${dockerfile}-tmp-e"
     fi
-
-    mv -f "${dockerfile}-tmp" "${dockerfile}"
+    if cmp -s "${dockerfile}-tmp" "${dockerfile}"; then
+      rm -f "${dockerfile}-tmp"
+      info "${dockerfile} already up to date."
+    else
+      mv -f "${dockerfile}-tmp" "${dockerfile}"
+      info "${dockerfile} updated."
+    fi
   )
 }