ci(portal-openbaar): serve the openbaar app in compose (nginx + BFF proxy) (refs #10)

Add the openbaar nginx image (serves the Angular app, reverse-proxies /openbaar to
the BFF, same-origin) and wire it into the compose stack on :8141, anonymous (no
Keycloak dependency). Add it to the health-wait list so the e2e can rely on it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/openbaar/Dockerfile

@@ -0,0 +1,21 @@
+# Multi-stage build for the openbaar portal (Angular → nginx).
+# Build context is the repo root (the app needs the pnpm workspace + libs). See infra/docker-compose.yml.
+FROM node:24-slim AS build
+WORKDIR /src
+RUN corepack enable && corepack prepare pnpm@11.5.2 --activate
+
+# Restore first (cached unless the manifests change).
+COPY package.json pnpm-lock.yaml pnpm-workspace.yaml nx.json tsconfig.base.json eslint.config.mjs ./
+RUN pnpm install --frozen-lockfile
+
+# Sources (only what the app + its libs need).
+COPY apps/openbaar apps/openbaar
+COPY libs libs
+RUN pnpm nx build openbaar
+
+FROM nginx:1.27-alpine AS runtime
+COPY apps/openbaar/nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /src/dist/apps/openbaar/browser /usr/share/nginx/html
+# No runtime config: the openbaar register is anonymous (no OIDC authority to inject).
+
+EXPOSE 80