delete containers during build after every step

This commit changes the way docker build cleans up containers.
Containers get cleaned up right away after they've been committed and
they've become an image.

When the build fails, only the last container of the failing step is
left behind.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Adding timestamp end to events endpoint. Modifying api docs.

Docker-DCO-1.1-Signed-off-by: Isabel Jimenez <contact@isabeljimenez.com> (github: jimenez)
Merge pull request #5140 from unclejack/build_immediate_cleanup

delete containers during build after every step
Merge pull request #4821 from jimenez/3903-add_event_end_timestamp-feature

Adding timestamp end to events endpoint. Modifying api docs.