fix(make): auto-detect docker/podman so Makefile works on either engine

- Makefile: introduce CONTAINER_CMD ?= $(shell command -v docker || echo podman),
  replace all 12 hardcoded `docker` invocations with $(CONTAINER_CMD). User can
  override with `make <target> CONTAINER_CMD=podman` or env export.
- e2e/tests/us1-bootstrap-admin.spec.ts: AC-1.4 regex updated to match the new
  variable form `$(CONTAINER_CMD) exec … flask create-admin` (was hardcoded
  `docker exec`). RUNTIME default also auto-detects (same logic as Makefile)
  so the test exec'es the right engine without a MIMIC_CONTAINER_CMD export.
- e2e/tests/us6-deployment.spec.ts: same RUNTIME auto-detect so the make-dry-run
  regex assertions on lines 75 + 77 match what the Makefile actually emits on
  a podman-only host.
- README + CHANGELOG document the new behavior.

Fixes the user-reported issue: "Le makefile ne fonctionne pas sur ma machine
qui n'a que podman."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -33,6 +33,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
 ### Changed
 - 2026-05-26 — `make update-mitre` upgraded from no-op placeholder to a real `curl` + optional container restart (Sprint 1 marker resolved).
 - 2026-05-26 — `EngagementDetailPage` no longer renders the "Simulations à venir au Sprint 2" placeholder; it embeds `<SimulationList>` instead.
+- 2026-05-26 — Makefile now auto-detects the container engine (`CONTAINER_CMD ?= docker || podman`) instead of hard-coding `docker`. Override with `make <target> CONTAINER_CMD=podman` or `export CONTAINER_CMD=…`. The matching e2e tests (`us1`, `us6`) were updated to mirror the same detection so they pass on podman-only machines without an explicit `MIMIC_CONTAINER_CMD` export.
 
 ---