WIP: add instructions to start a Singularity container inside the cepc/cepcsw Docker container.

Dockerfile

@@ -25,6 +25,17 @@ RUN sudo yum update -y && \
     sudo yum update -y && \
     sudo yum install -y singularity
 
+# If the CVMFS is installed in the host, just mount the corresonding /CVMFS directories
+# $ docker run --privileged --rm -i -t \
+           -v /cvmfs/sft.cern.ch:/cvmfs/sft.cern.ch \
+           -v /cvmfs/cepcsw.ihep.ac.cn:/cvmfs/cepcsw.ihep.ac.cn \
+           -v /cvmfs/container.ihep.ac.cn:/cvmfs/container.ihep.ac.cn \
+           cepc/cepcsw /bin/bash
+# Inside the Docker container, we could start the Singularity provided by IHEP
+# $ export SINGULARITY_BINDPATH=/cvmfs
+# $ singularity shell /cvmfs/container.ihep.ac.cn/singularity/image/SL69/sl69worknode20200729.sif
+
+##############################################################################
 # Install CVMFS
 # Configure IHEP
 # Enable /cvmfs/cepcsw.ihep.ac.cn and /cvmfs/container.ihep.ac.cn