Up until now, chained containers would execute one after the other as
long as they would have the exit code 0. Let's change that to ignoring
the exit code, and allow the user to set b2c.pipefail to restore the
previous behaviour.

This is to mimic what shell scripts are doing, and thus be less of a
surprise to users.

Suggested-by: Charlie Turner <cturner@igalia.com>