diff --git a/1.6/Dockerfile b/1.6/Dockerfile
index 302b270..956489d 100644
--- a/1.6/Dockerfile
+++ b/1.6/Dockerfile
@@ -1,4 +1,4 @@
-FROM php:7.3.19-fpm-buster
+FROM php:7.3.19-fpm-buster@sha256:998f3dc664d570103af88044215b347bff77c43de95c52be1ebae0a0cfd08543
 
 ENV GRAV_VERSION 1.6.26
 
diff --git a/1.7/Dockerfile b/1.7/Dockerfile
index 121be76..cce2c4c 100644
--- a/1.7/Dockerfile
+++ b/1.7/Dockerfile
@@ -1,4 +1,4 @@
-FROM php:7.3.19-fpm-buster
+FROM php:7.3.19-fpm-buster@sha256:998f3dc664d570103af88044215b347bff77c43de95c52be1ebae0a0cfd08543
 
 ENV GRAV_VERSION 1.7.0-rc.12