Fix silent config corruption in 50-ipv6.sh on NFS volumes

Replace unsafe `echo "$(sed ...)" > $FILE` with atomic temp-file write.

The current pattern reads a file with sed inside a command substitution,
then writes the result back via echo redirection. If sed reads an empty
or momentarily unreadable file (e.g., NFS transient issue during
container recreation by Watchtower or similar tools), it produces no
output. The echo then writes exactly 1 byte (a newline) to the config
file, silently destroying its contents.

The fix writes sed output to a temp file first, checks it's non-empty
with `[ -s ]`, then atomically replaces the original via `mv`. If sed
produces empty output, the original file is preserved and a warning is
logged to stderr.

docker/rootfs/etc/s6-overlay/s6-rc.d/prepare/50-ipv6.sh

@@ -25,7 +25,13 @@ process_folder () {
 	for FILE in $FILES
 	do
 		echo "- ${FILE}"
-		echo "$(sed -E "$SED_REGEX" "$FILE")" > $FILE
+		TMPFILE="${FILE}.tmp"
+		if sed -E "$SED_REGEX" "$FILE" > "$TMPFILE" && [ -s "$TMPFILE" ]; then
+			mv "$TMPFILE" "$FILE"
+		else
+			echo "WARNING: skipping ${FILE} — sed produced empty output" >&2
+			rm -f "$TMPFILE"
+		fi
 	done
 
 	# ensure the files are still owned by the npm user