Only "created" containers

[rpatel-vt]

In our scenario we use multistage docker builds and intermediate containers are not created are never executed. In those scenario we would like to keep those containers till GRACE_PERIOD_SECONDS.

[davidxia]

Thanks for your PR. Could you add an env var EXCLUDE_INTERMEDIATE_CONTAINERS that defaults to false and use that in the conditional? This way the behavior won't change and will be opt in for existing users.