Fix docker exec command to docker compose exec

[jmoses]

This ensure the exec call is scoped to the current compose containers, instead of potentially hitting another container named 'aura', unlikely as that is.

[promptless]

Promptless prepared a documentation update related to this change.

Triggered by PR #293

The same stale docker exec references appeared in the Aura docs, so this update corrects them to docker compose exec for consistency — the quickstart command, the architecture diagram label, and the prose reference.

Review: Use docker compose exec for aura-cli in the quickstart