Reorder dockerfile

This creates more intermediates, but moves those that are less likely
to change to the top of the file. In theory this will produce faster
builds on a developer's machine as they won't need to apt-get update
every time.

docker/Dockerfile

@@ -1,6 +1,8 @@
 FROM python:3.6
 
-COPY constraints.txt requirements.txt /reqs/
+
+RUN set -ex \
+    && apt-get update
 
 RUN set -ex \
     && buildDeps=' \
@@ -15,12 +17,18 @@ RUN set -ex \
         libmemcached-dev \
         libsasl2-dev \
     ' \
-    && apt-get update \
+
     && apt-get install -y git xmlsec1 libmysqlclient18 \
     && apt-get install -y $buildDeps --no-install-recommends \
-    && rm -rf /var/lib/apt/lists/* \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN set -ex \
+    && pip install uwsgi
+
+COPY constraints.txt requirements.txt /reqs/
+
+RUN set -ex \
     && pip install --no-cache-dir -r /reqs/requirements.txt -c /reqs/constraints.txt \
-    && pip install uwsgi \
     && apt-get purge -y --auto-remove $buildDeps \
     && rm -rf /usr/src/python ~/.cache