The default values of masked and readonly paths are defined
in populateDefaultUnixSpec, and are used when a sandbox is
created. It is not, however, used for new containers. If
a container definition does not contain a security context
specifying masked/readonly paths, a container created from
it does not have masked and readonly paths.
This patch applies the default values to masked and
readonly paths of a new container, when any specific values
are not specified.
Fixes#1569
Signed-off-by: Yohei Ueda <yohei@jp.ibm.com>