Add Dockerfile for sequence diagram generation

docs/design/clustering/Dockerfile

@@ -0,0 +1,12 @@
+FROM debian:jessie
+
+RUN apt-get update
+RUN apt-get -qy install python-seqdiag make curl
+
+WORKDIR /diagrams
+
+RUN curl -sLo DroidSansMono.ttf https://googlefontdirectory.googlecode.com/hg/apache/droidsansmono/DroidSansMono.ttf
+
+ADD . /diagrams
+
+CMD bash -c 'make >/dev/stderr && tar cf - *.png'