hotfix(Dockerfile): fix approvaltest dependency

As approvaltest 3.1.1 seems to be flawed, we're falling back go 3.1.0,
which brings in another dependency on empty-files. Should be obsolete
when switching to a newer version.
Signed-off-by: Rene Paris <rene.paris@in-tech.com>

utils/Dockerfile

@@ -34,7 +34,8 @@ RUN apt-get install -qq -y --no-install-recommends \
     texlive-latex-extra
 
 RUN pip install \
-    approvaltests \
+    approvaltests==3.1.0 \
+    empty-files \
     breathe \
     conan \
     exhale \