This fix tries to address the issue raised in 28769 where
checkpoint name was not checked before passing to containerd.
As a result, it was possible to use a special checkpoint name
to get outside of the container's directory.
This fix add restriction `[a-zA-Z0-9][a-zA-Z0-9_.-]+` (`RestrictedNamePattern`).
This is the same as container name restriction.
This fix fixes 28769.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
