feat(infra): containerize BFF + compose-up smoke (closes #29) (#36)

services/bff/.dockerignore

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+**/bin
+**/obj
+**/*.user

services/bff/Dockerfile

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+# 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"]