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>

README.md

@@ -56,6 +56,15 @@ docker compose -f infra/docker-compose.yml up -d
 
 Health checks should be green within ~3 minutes on a developer machine. If something fails, see [docs/runbooks/local-startup.md](docs/runbooks/local-startup.md).
 
+> **Wired today (Iteration 0):** only the placeholder BFF is in `infra/docker-compose.yml` so far. Bring it up and smoke-test its health endpoint:
+>
+> ```bash
+> docker compose -f infra/docker-compose.yml up -d --build --wait
+> curl http://localhost:8080/health   # -> Healthy
+> ```
+>
+> `--wait` exits non-zero unless the container reports healthy, so this doubles as the compose-up smoke test. The remaining services and the URLs below land in later slices.
+
 **Default URLs**
 
 | Service | URL |

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

services/bff/.dockerignore

@@ -0,0 +1,3 @@
+**/bin
+**/obj
+**/*.user

services/bff/Dockerfile

@@ -0,0 +1,30 @@
+# Multi-stage build for the placeholder BFF (.NET 10).
+# Build context is services/bff (see infra/docker-compose.yml).
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+
+# Restore first (cached unless the csproj changes).
+COPY Bff.Api/Bff.Api.csproj Bff.Api/
+RUN dotnet restore Bff.Api/Bff.Api.csproj
+
+# Then build + publish.
+COPY Bff.Api/ Bff.Api/
+RUN dotnet publish Bff.Api/Bff.Api.csproj -c Release -o /app/publish /p:UseAppHost=false
+
+FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
+WORKDIR /app
+
+# curl is used by the container HEALTHCHECK / compose healthcheck.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=build /app/publish .
+
+ENV ASPNETCORE_URLS=http://+:8080
+EXPOSE 8080
+
+HEALTHCHECK --interval=5s --timeout=3s --start-period=10s --retries=5 \
+    CMD curl -fsS http://localhost:8080/health || exit 1
+
+ENTRYPOINT ["dotnet", "Bff.Api.dll"]