additional user in slurm base docker image

roles/cobald/templates/cobald.Dockerfile

@@ -11,7 +11,8 @@ RUN git clone $REPOCOBALD /usr/local/src/cobald && \
 	git clone $REPOTARDIS /usr/local/src/tardis
 
 RUN mkdir /etc/cobald /var/log/cobald && \
-	useradd -m -d /var/lib/cobald --no-log-init --system cobald && \
+	( getent passwd cobald > /dev/null || \
+		useradd -m -d /var/lib/cobald --no-log-init --system cobald ) && \
 	chown cobald:cobald /var/log/cobald
 
 #RUN mkdir /cobald && python3 -m venv /cobald && source /cobald/bin/activate &&\