Re-use helper container for docker executor's predefined stages

What does this MR do?

This changes predefined steps to reuse the same container, rather than create a new one each time. This is similar to how build steps are already being performed with a build container.

Now that there's only ever one predefined container per job, this change also removes the additional book-keeping that was required to append an index to the container names.

Why was this MR needed?

• Removes redundant e.builds book-keeping (#25301 (closed)).
• Slightly - more than slightly on Windows - improves performance by reducing the amount of containers needing to be created and destroyed.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #25301 (closed)