Dockerfile: add imagemagick

In reStructured text documentation we sometimes find formatting problems.
So we should 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 ImageMagick
package.
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

Dockerfile

@@ -53,6 +53,7 @@ RUN apt-get update && apt-get install -y \
 	grub-efi-amd64-bin \
 	grub-efi-ia32-bin \
 	iasl \
+	imagemagick \
 	iputils-ping \
 	libisl15 \
 	liblz4-tool \