Fix missing dependency warning

Thank you @AnotherFoxGuy for figuring this one out in https://github.com/netlify/build-image/pull/209/files

Author: bcomnes

Dockerfile

@@ -107,7 +107,7 @@ RUN export DEBIAN_FRONTEND=noninteractive && \
         libgif-dev \
         libglib2.0-dev \
         libgmp3-dev \
-        libgraphicsmagick3 \
+        libgraphicsmagick-q16-3 \
         libgtk-3-0 \
         libgtk2.0-0 \
         libicu-dev \