back up erlang cookie on release upgrade

rel/files/bin/pleroma_ctl

@@ -86,6 +86,9 @@ update() {
 	curl "$full_uri" -o "${artifact}"
 	echo "Unpacking ${artifact} to ${tmp}"
 	unzip -q "$artifact" -d "$tmp"
+    echo "Backing up erlang cookie"
+    erlang_cookie=$(cat $RELEASE_ROOT/releases/COOKIE)
+    echo "Cookie: $erlang_cookie"
 	echo "Copying files over to $RELEASE_ROOT"
 	if [ "$NO_RM" = false ]; then
 		echo "Removing files from the previous release"
@@ -95,6 +98,8 @@ update() {
 	echo "Removing temporary files"
 	rm -r "$tmp/release"
 	rm "$artifact"
+    echo "Restoring erlang cookie"
+    echo $erlang_cookie > $RELEASE_ROOT/releases/COOKIE
 	echo "Done! Please refer to the changelog/release notes for changes and update instructions"
 	set +e
 }