Fixed cert script

Author: mmaehren

certs/Dockerfile

@@ -1,5 +1,8 @@
-FROM debian:sid
+FROM debian:stretch
 
+RUN apt-get update 
+RUN apt-get install software-properties-common -y
+RUN apt-add-repository 'deb http://security.debian.org/debian-security stretch/updates main'
 RUN apt-get update && apt-get install -y \
     openssl \
     libnss3-tools \