feat(portal-self-service): runtime config + nginx serve/proxy image (refs #68)

The app loads /config.json at startup (main.ts) so the OIDC authority is set per
environment from one build; appConfig becomes a factory and derives redirectUrl +
secureApiOrigin from the app origin (same-origin as the BFF). A multi-stage
Dockerfile builds the app and serves it via nginx, reverse-proxying /self-service
+ /openbaar to the bff (relative URLs → no CORS); nginx resolves the BFF at request
time. The compose image bakes config.json with the keycloak:8080 authority so the
browser's token issuer matches the BFF (ADR-0010).

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

apps/self-service/Dockerfile

@@ -0,0 +1,23 @@
+# Multi-stage build for the self-service 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/self-service apps/self-service
+COPY libs libs
+RUN pnpm nx build self-service
+
+FROM nginx:1.27-alpine AS runtime
+COPY apps/self-service/nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /src/dist/apps/self-service/browser /usr/share/nginx/html
+# Compose-time OIDC config: the browser (Playwright, on the compose network) reaches Keycloak by
+# service name, so the token issuer matches the BFF's authority (host-consistent, ADR-0010).
+RUN printf '{ "authority": "http://keycloak:8080/realms/digid" }\n' > /usr/share/nginx/html/config.json
+
+EXPOSE 80