Add docs test/build

Integrate the doc build in the new secop job

TODO: Check the publishing part

Change-Id: I361ec085aad04b5155bb3f972b329dd2a7445aaa
Reviewed-on: https://forge.frm2.tum.de/review/18661
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Bjoern Pedersen <bjoern.pedersen@frm2.tum.de>

ci/Dockerfile

@@ -7,6 +7,7 @@ RUN apt-get update && \
           git \
           openssh-client \
           make \
+          locales \
           python \
           python-dev \
           python-virtualenv \
@@ -33,10 +34,13 @@ RUN virtualenv /home/jenkins/secopvenv && \
 FROM base AS docs
 ENV  DEBIAN_FRONTEND=noninteractive
 
+USER root
 RUN apt-get update && \
    apt-get install -y --no-install-suggests --no-install-recommends \
      texlive-latex-extra \
      texlive-latex-base \
      texlive-latex-recommended \
+     texlive-fonts-recommended \
      texlive-base \
-     texlive-binaries
+     texlive-binaries \
+     latexmk