Modify dependencies to suit debian stretch

The python:3.6 tag now points to stretch.

Changed the name of the base image to make this more explicit.

docker/Dockerfile

@@ -1,11 +1,9 @@
-FROM python:3.6 as symposion_base
+FROM python:3-stretch as symposion_base
 
 RUN set -ex \
     && apt-get update
-
 RUN set -ex \
     && buildDeps=' \
-        libmysqlclient-dev \
         libffi-dev \
         libfreetype6-dev \
         libjpeg-dev \
@@ -16,7 +14,7 @@ RUN set -ex \
         libmemcached-dev \
         libsasl2-dev \
     ' \
-    && apt-get install -y git xmlsec1 libmysqlclient18 \
+    && apt-get install -y git xmlsec1 \
     && apt-get install -y $buildDeps --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*