Add pkg-config to docs/Dockerfile
This prevents a build error.
@@ -33,6 +33,7 @@ RUN apt-get update && apt-get install -y -q --no-install-recommends \
RUN apt-get update && apt-get install -y -q \
dvipng \
make \
+ pkg-config \
sudo \
texlive-fonts-recommended \
texlive-latex-base \
