[Build]: Fix /proc not mounted issue

[xumia]

Why I did it

It does not make sense to unmount /proc in the slave container.

It has impact multiple programs, for example:

root@8f495bfec510:/# umount /proc
root@8f495bfec510:/# ps -a       
Error, do this: mount -t proc proc /proc

Another example, the slave container will become unreachable when /proc unmounted.

$ docker exec 8f495bfec510 uname -m
OCI runtime exec failed: exec failed: container_linux.go:346: starting container process caused "close exec fds: open /proc/self/fd: no such file or directory": unknown

How I did it

How to verify it

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[qiluo-msft]

LGTM. Please wait a while for other reviewers.

[qiluo-msft]

This commit could not be cleanly cherry-picked to 202012. Please submit another PR.

[xumia]

This commit could not be cleanly cherry-picked to 202012. Please submit another PR.

@qiluo-msft , thanks for the notice, I have sent a new PR #10256 to fix the conflict.