util/docker/coreboot-jenkins-node: Install cmocka

flashrom uses cmocka for unit testing. Install it so that the CI can
use it.

Change-Id: I5c168e480d6f4cbfbbd175ecb035c88bfcbac00b
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67272
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/docker/coreboot-jenkins-node/Dockerfile

@@ -25,6 +25,7 @@ USER root
 RUN apt-get -y update && \
 	apt-get -y install \
 		default-jre-headless \
+		libcmocka-dev \
 		liblua5.3-dev \
 		linkchecker \
 		lua5.3 \