Missing python module

[patrickhwood]

Had to make this change to Dockerfile to get the swift-proxy-server to run in my build:

diff --git a/Dockerfile b/Dockerfile
index 0949207..b55ea80 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -25,6 +25,8 @@ RUN apt-get install -y \
     libxml2-dev \
     libxslt1-dev

+RUN pip install lxml
+
 # to get a decently recent version of swift3, you have to git clone...
 RUN cd /root; git clone https://github.com/stackforge/swift3.git
 RUN cd /root/swift3; python setup.py install