Refactor docker locations and README.

This addresses #3224

docker/Dockerfile

@@ -0,0 +1,35 @@
+FROM docker.io/python:2-alpine3.7
+
+RUN apk add --no-cache --virtual .nacl_deps \
+        build-base \
+        libffi-dev \
+        libjpeg-turbo-dev \
+        libressl-dev \
+        libxslt-dev \
+        linux-headers \
+        postgresql-dev \
+        su-exec \
+        zlib-dev
+
+COPY . /synapse
+
+# A wheel cache may be provided in ./cache for faster build
+RUN cd /synapse \
+ && pip install --upgrade \
+        lxml \
+        pip \
+        psycopg2 \
+        setuptools \
+ && mkdir -p /synapse/cache \
+ && pip install -f /synapse/cache --upgrade --process-dependency-links . \
+ && mv /synapse/docker/start.py /synapse/docker/conf / \
+ && rm -rf \
+        setup.cfg \
+        setup.py \
+        synapse
+
+VOLUME ["/data"]
+
+EXPOSE 8008/tcp 8448/tcp
+
+ENTRYPOINT ["/start.py"]