Add lxml to docker image for web previews

Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>

Dockerfile

@@ -1,12 +1,12 @@
 FROM docker.io/python:2-alpine3.7
 
-RUN apk add --no-cache --virtual .nacl_deps su-exec build-base libffi-dev zlib-dev libressl-dev libjpeg-turbo-dev linux-headers postgresql-dev
+RUN apk add --no-cache --virtual .nacl_deps su-exec build-base libffi-dev zlib-dev libressl-dev libjpeg-turbo-dev linux-headers postgresql-dev libxslt-dev
 
 COPY . /synapse
 
 # A wheel cache may be provided in ./cache for faster build
 RUN cd /synapse \
- && pip install --upgrade pip setuptools psycopg2 \
+ && pip install --upgrade pip setuptools psycopg2 lxml \
  && mkdir -p /synapse/cache \
  && pip install -f /synapse/cache --upgrade --process-dependency-links . \
  && mv /synapse/contrib/docker/start.py /synapse/contrib/docker/conf / \