Task 0.1 of the s6-overlay supervision plan. Establishes the test
infrastructure for tests/docker/: skip-on-missing-Docker collection
hook, session-scoped image-build fixture (overridable via the
HERMES_TEST_IMAGE env var for faster local iteration), and a
container_name fixture that ensures cleanup on test exit.
Refs: docs/plans/2026-05-07-s6-overlay-dynamic-subagent-gateways.md
