Add the file package for www Docker image

Needed for https://gitlab.com/gitlab-com/www-gitlab-com/issues/1150

Dockerfile.www-gitlab-com

 FROM ruby:2.3-slim
-ENV PACKAGES="git-core pandoc texlive-latex-recommended texlive-xetex texlive-fonts-recommended lmodern ed"
+ENV PACKAGES="git-core pandoc texlive-latex-recommended texlive-xetex texlive-fonts-recommended lmodern ed file"
 ADD / /
 RUN /scripts/install-www-gitlab-com