contrib/ci: install rsync on jenkins-community-slaves

Rsync is a requirement for OpenWrt master and therefore our next
branch. Currently builds on x86-64 error out due to missing rsync:

/bin/sh: 1: rsync: not found

contrib/ci/jenkins-community-slave/Dockerfile

@@ -5,7 +5,7 @@ USER root
 # this is needed to install default-jre-headless in debian slim images
 RUN mkdir -p /usr/share/man/man1
 
-RUN apt-get update && apt-get install -y default-jre-headless curl git netcat-openbsd python3 python3-pip qemu-system-x86 iproute2 openssh-client
+RUN apt-get update && apt-get install -y default-jre-headless curl git netcat-openbsd python3 python3-pip qemu-system-x86 iproute2 openssh-client rsync
 RUN python3 -m pip install jenkins-webapi sphinx sphinx_rtd_theme gluon-qemu-testlab==0.0.5
 
 # Get docker-compose in the agent container