Dockerfile: add graphviz

In reStructured text documentation we somtimes find formatting problems.
Therefore I would like to add 'make htmldocs' to our Gitlab Ci steps and
let all warning result in a build failure.

For building the htmldocs target without warnings we need the Graphviz
package.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

Dockerfile

@@ -49,6 +49,7 @@ RUN apt-get update && apt-get install -y \
 	flex \
 	gdisk \
 	git \
+	graphviz \
 	grub-efi-amd64-bin \
 	grub-efi-ia32-bin \
 	iasl \