[feature request] If the mount destination is not empty, we should see all the content after mounting

[YaoZengzeng]

Ⅰ. Issue Description

The Docker file of image quay.io/calico/cni:v3.1.3 is :

FROM busybox

LABEL maintainer "Tom Denham <tom@tigera.io>"

ADD dist/amd64/calico /opt/cni/bin/calico
ADD dist/amd64/flannel /opt/cni/bin/flannel
ADD dist/amd64/loopback /opt/cni/bin/loopback
ADD dist/amd64/host-local /opt/cni/bin/host-local
ADD dist/amd64/portmap /opt/cni/bin/portmap
ADD dist/amd64/calico-ipam /opt/cni/bin/calico-ipam
ADD k8s-install/scripts/install-cni.sh /install-cni.sh
ADD k8s-install/scripts/calico.conf.default /calico.conf.tmp

ENV PATH=$PATH:/opt/cni/bin
VOLUME /opt/cni
WORKDIR /opt/cni/bin
CMD ["/opt/cni/bin/calico"]

The result of running this image with Pouch and Docker is different:

[root@cal pouch]# docker run -it -v /opt/cni/bin quay.io/calico/cni:v3.1.3 sh
/opt/cni/bin # ls
calico       calico-ipam  flannel      host-local   loopback     portmap

[root@cal pouch]# pouch run -it -v /opt/cni/bin quay.io/calico/cni:v3.1.3 sh
/opt/cni/bin # ls
/opt/cni/bin # 

The reason of the difference is that the volume destination of /opt/cni is not empty, it has subdirectory cni in it.

After mounting, Docker will copy the subdirectory cni to the mount destination and Pouch has not supported this feature, we should implement this ASAP.

Ⅱ. Describe what happened

Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

[pouchrobot]

Thanks for your report, @YaoZengzeng
😱 This is a priority/P1 issue which is highest.
Seems to be severe enough.
ping @alibaba/pouch , PTAL.