feat(infra): containerize BFF + compose-up smoke (refs #29)

Add a multi-stage Dockerfile for the BFF (.NET 10 sdk -> aspnet runtime,
curl for the healthcheck) and infra/docker-compose.yml running it with a
/health healthcheck. `docker compose up --wait` gates on the container
reporting healthy, which is the compose-up smoke test. Document the path
in the README.

Verified: image builds; `docker compose up -d --build --wait` reports the
container Healthy; host curl http://localhost:8080/health -> 200 Healthy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker-compose.yml

@@ -0,0 +1,19 @@
+# Local development stack. Grows service-by-service with each slice.
+# S-00-b: the placeholder BFF with a /health check.
+#
+#   docker compose -f infra/docker-compose.yml up -d --build --wait
+#   curl http://localhost:8080/health   # -> Healthy
+services:
+  bff:
+    build:
+      context: ../services/bff
+      dockerfile: Dockerfile
+    image: register-referentie/bff:dev
+    ports:
+      - "8080:8080"
+    healthcheck:
+      test: ["CMD", "curl", "-fsS", "http://localhost:8080/health"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s