diff --git a/modules/restricted-guests/synapse/Dockerfile b/modules/restricted-guests/synapse/Dockerfile
index dc39a088c6..1e1d1e557c 100644
--- a/modules/restricted-guests/synapse/Dockerfile
+++ b/modules/restricted-guests/synapse/Dockerfile
@@ -1,7 +1,7 @@
 ARG DEBIAN_VERSION_NUMERIC=12
 
 # Now copy it into our base image.
-FROM gcr.io/distroless/base-nossl-debian${DEBIAN_VERSION_NUMERIC}:debug AS build
+FROM gcr.io/distroless/base-nossl-debian${DEBIAN_VERSION_NUMERIC}:debug@sha256:12dbb4f46c5f712fe3da1c7a441602ee91eb87a5d46b0e725b4440b852000538 AS build
 
 FROM gcr.io/distroless/base-nossl-debian${DEBIAN_VERSION_NUMERIC}