CI: Mounting volumes from a service image in the main build image

I have a CI build for a test, where the test writes some log message to the syslog via rsyslogd. Since rsyslogd is a process that accepts log messages via a socket, I want to run it as a service in its own container. Then, the test reads the log from /var/log/syslog from the rsyslogd image (specifically, I use kamermans/rsyslog).

When running the test image locally, I can use --volumes-from rsyslogd to gain access to the syslog. How can I do this when rsyslogd is running as a job service?