Add graphviz in Dockerfile

Author: MaxDesiatov

Dockerfile

@@ -6,7 +6,7 @@ RUN mkdir -p /build/lib && cp -R /usr/lib/swift/linux/*.so* /build/lib
 RUN make install prefix=/build
 
 FROM ubuntu:18.04
-RUN apt-get -qq update && apt-get install -y libatomic1 libxml2-dev libcurl4-openssl-dev && rm -r /var/lib/apt/lists/*
+RUN apt-get -qq update && apt-get install -y graphviz libatomic1 libxml2-dev libcurl4-openssl-dev && rm -r /var/lib/apt/lists/*
 COPY --from=builder /build/bin/swift-doc /usr/bin
 COPY --from=builder /build/lib/* /usr/lib/
 ENTRYPOINT ["swift-doc"]