Install libxml2-dev on builder image

Author: mattt

Dockerfile

@@ -1,11 +1,12 @@
 FROM swift:5.2 as builder
 WORKDIR /swiftdoc
 COPY . .
+RUN apt-get -qq update && apt-get install -y libxml2-dev && rm -r /var/lib/apt/lists/*
 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 && rm -r /var/lib/apt/lists/*
+RUN apt-get -qq update && apt-get install -y libatomic1 && 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"]