Modify the tag version from latest to 2018.03.0.20180424. Because latest became Amazon linux 2.

keillera · keillera · commit 5054e5087ce3 · 2018-06-27T16:00:02.000+09:00
diff --git a/Dockerfile b/Dockerfile
@@ -1,4 +1,4 @@
-FROM amazonlinux:latest
+FROM amazonlinux:2018.03.0.20180424
 
 WORKDIR /workdir
 COPY requirements.txt ./
