diff --git a/deploy/Dockerfile b/deploy/Dockerfile
index 870e6ad3b..330e44ab5 100644
--- a/deploy/Dockerfile
+++ b/deploy/Dockerfile
@@ -12,7 +12,7 @@ RUN yarn run build --env production
 # → /src/_output/UI
 
 # ---------- Backend ----------
-FROM mcr.microsoft.com/dotnet/sdk:6.0 AS backend
+FROM mcr.microsoft.com/dotnet/sdk:9.0 AS backend
 WORKDIR /src
 COPY .editorconfig ./
 COPY src ./src