One of the logging options for Scylla is syslog, this method, until today wasn't supported in the docker images that are created with the Dockerfile in the repo. This commit add rsyslog installation, configuration and setup for Docker. Tests: built and ran the docker and validated the existance of the /dev/log socket. Signed-off-by: Eliran Sinvani <eliransin@scylladb.com> Message-Id: <20200210112448.210169-1-eliransin@scylladb.com>