Update Dockerfile to have mapnik backported

Dockerfile

@@ -1,5 +1,8 @@
 FROM debian:stretch
-RUN apt-get update && apt-get install -y nodejs nodejs-legacy git wget build-essential python libcairo2-dev libgif-dev libpango1.0-dev libjpeg62-turbo-dev apt-transport-https redis-server libmapnik-dev && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y nodejs nodejs-legacy git wget python apt-transport-https redis-server libboost-filesystem-dev libboost-program-options-dev libboost-regex-dev libboost-system-dev libboost-thread-dev libxml2 libxml2-dev libfreetype6 libfreetype6-dev libjpeg-dev libpng-dev libtiff5-dev libcairo2 libcairo2-dev ttf-unifont ttf-dejavu ttf-dejavu-core ttf-dejavu-extra build-essential libgdal-dev libharfbuzz-bin libharfbuzz-dev libmapnik-dev && rm -rf /var/lib/apt/lists/*
+RUN printf "deb https://apt.wikimedia.org/wikimedia stretch-wikimedia main backports" > /etc/apt/sources.list
+RUN wget -qO - "https://wikitech.wikimedia.org/w/index.php?title=APT_repository/Stretch-Key&action=raw" | apt-key add -
+RUN apt-get update && apt-get install -y -t stretch-wikimedia libmapnik3.0
 ENV NVM_DIR /usr/local/nvm
 RUN wget -qO- https://raw.githubusercontent.com/creationix/nvm/v0.33.6/install.sh | bash && . $NVM_DIR/nvm.sh && nvm install 6.11.1
 ENV HOME=/root/ LINK=g++