fix docker apt repository for stretch

Dockerfile

@@ -15,7 +15,7 @@ RUN apt-get update -qq && apt-get install -qqy \
     netcat-openbsd \
     ssmtp \
  && curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add - \
- && echo "deb [arch=amd64] https://download.docker.com/linux/debian jessie stable" >> /etc/apt/sources.list \
+ && echo "deb [arch=amd64] https://download.docker.com/linux/debian stretch stable" >> /etc/apt/sources.list \
  && apt-get update -qq && apt-get install -qqy docker-ce
 
 RUN pip3 install --no-cache-dir \