diff --git a/Dockerfile b/Dockerfile
index e711dd4..d225daf 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ubuntu:latest
+FROM ubuntu:bionic
 LABEL maintainer="Xiaonan Shen <s@sxn.dev>"
 
 EXPOSE 25/tcp
diff --git a/build/Dockerfile b/build/Dockerfile
index b579920..bdec4ef 100644
--- a/build/Dockerfile
+++ b/build/Dockerfile
@@ -8,7 +8,7 @@ WORKDIR /build/
 COPY build.sh VERSION /build/
 RUN bash build.sh
 
-FROM ubuntu:latest
+FROM ubuntu:bionic
 LABEL maintainer="Xiaonan Shen <s@sxn.dev>"
 
 EXPOSE 25/tcp