Remove e.builds from Docker executor

Overview

We use e.builds to keep track of the containers that we created.

We keep track of the containers because of two reasons:

1. To we create a unique index for the predefined containers
2. To use that container for volumesFrom to get volume definitions.

Both of these are getting removed in:

1. !1801 (merged)
2. !1989 (merged)

Proposal

Since we no longer need to keep track of the containers that we create thanks to !1801 (merged) & !1989 (merged) we should remove the need for e.builds