diff --git a/fpm-alpine/Dockerfile b/fpm-alpine/Dockerfile
index 174db00..7c295bc 100644
--- a/fpm-alpine/Dockerfile
+++ b/fpm-alpine/Dockerfile
@@ -1,4 +1,4 @@
-FROM php:8.1-fpm-alpine as root
+FROM php:8.1-fpm-alpine3.21 as root
 LABEL maintainer="Thomas Bruederli <thomas@roundcube.net>"
 LABEL org.opencontainers.image.source="https://github.com/roundcube/roundcubemail-docker"
 
diff --git a/templates/Dockerfile-alpine.templ b/templates/Dockerfile-alpine.templ
index e6fd88c..bf2acab 100644
--- a/templates/Dockerfile-alpine.templ
+++ b/templates/Dockerfile-alpine.templ
@@ -1,4 +1,4 @@
-FROM php:8.1-%%VARIANT%% as root
+FROM php:8.1-%%VARIANT%%3.21 as root
 LABEL maintainer="Thomas Bruederli <thomas@roundcube.net>"
 LABEL org.opencontainers.image.source="https://github.com/roundcube/roundcubemail-docker"