feat(backend): optional lean deployable image (WP-30 #5)

Multi-stage backend/Dockerfile (sdk build -> aspnet:10.0 runtime, ~312MB) +
docker-compose.prod.yml, additive only — not wired into CI or the existing
dev docker-compose.yml (which keeps the SDK image for dotnet run hot-reload).
New .dockerignore keeps the build context lean (node_modules alone is
~750MB) since the Dockerfile COPYs from the repo root to pick up
public/letter.css (WP-25's FE<->BE letter contract) as a sibling of backend/.

Verified for real: built the image, ran it, and curled a live
GET /api/v1/brief/preview against the running container — got back the
actual rendered letter HTML with letter.css inlined, confirming the
walk-up-from-BaseDirectory lookup resolves inside this image layout too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,12 @@
+# WP-30: backend/Dockerfile does `COPY . .` from the repo root (needs public/letter.css as a
+# sibling of backend/) — without this, that copy would drag in node_modules (~750MB), .git,
+# and every build/output directory, defeating the point of a "lean" image.
+.git
+node_modules
+dist
+storybook-static
+backend/**/bin
+backend/**/obj
+backend/**/bigregister.db*
+backend/openzaak
+documentation.json

backend/Dockerfile

@@ -0,0 +1,15 @@
+# WP-30: lean deployable image (optional — not used by the dev demo, which keeps the SDK
+# image in the root docker-compose.yml for `dotnet run` hot-reload). Build from the repo
+# root: `docker build -f backend/Dockerfile .`
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+COPY . .
+RUN dotnet publish backend/src/BigRegister.Api -c Release -o /app
+
+FROM mcr.microsoft.com/dotnet/aspnet:10.0
+WORKDIR /app
+COPY --from=build /app .
+# LetterHtml.Render (WP-25) walks up from AppContext.BaseDirectory looking for a sibling
+# public/letter.css (the FE⇄BE letter contract) — this keeps that lookup working here too.
+COPY public ./public
+ENTRYPOINT ["dotnet", "BigRegister.Api.dll"]

docker-compose.prod.yml

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+# WP-30: optional lean deployable backend image — NOT used by the dev demo (docker-compose.yml
+# keeps the SDK image there for `dotnet run` hot-reload; rewriting that one into a prod image
+# would lose the bind-mount hot-reload it's for). This is an additive artifact only, not wired
+# into CI. `docker compose -f docker-compose.prod.yml up --build` → Swagger at :5000/swagger.
+services:
+  api:
+    build:
+      context: .
+      dockerfile: backend/Dockerfile
+    environment:
+      - ASPNETCORE_ENVIRONMENT=Production
+      - ASPNETCORE_URLS=http://+:8080
+    ports:
+      - '5000:8080'