Switch from Caddy templates to runtime envsubst for API and Keycloak URLs, adjust Dockerfile and entrypoint script

config/docker/caddy/web-app/Dockerfile

@@ -13,6 +13,9 @@ ARG BUILD_DATE
 # ===================================================================
 FROM caddy:${CADDY_VERSION}
 
+# envsubst für Env-Var-Substitution zur Laufzeit
+RUN apk add --no-cache gettext
+
 ARG VERSION
 ARG BUILD_DATE
 
@@ -21,13 +24,17 @@ LABEL service="web-app" \
       maintainer="Meldestelle Development Team" \
       build.date="${BUILD_DATE}"
 
-# Copy Caddy Config & Templates
+# Copy Caddy Config & Entrypoint
 COPY config/docker/caddy/web-app/Caddyfile /etc/caddy/Caddyfile
 COPY config/docker/caddy/web-app/config.json /usr/share/caddy/config.json
+COPY config/docker/caddy/web-app/entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
 
 # Copy Pre-built Static Assets from Host
 # NOTE: You must run `./gradlew :frontend:shells:meldestelle-portal:jsBrowserDistribution -Pproduction=true` locally first!
 COPY frontend/shells/meldestelle-portal/build/dist/js/productionExecutable/ /usr/share/caddy/
+# index.html wird als Template abgelegt; der Entrypoint erzeugt daraus zur Laufzeit die finale index.html
+RUN mv /usr/share/caddy/index.html /usr/share/caddy/index.html.tmpl
 
 # Ensure favicon exists (fallback)
 # Using the shared asset from existing config structure
@@ -38,4 +45,5 @@ EXPOSE 4000
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
     CMD wget --no-verbose --tries=1 --spider http://localhost:4000/health || exit 1
 
+ENTRYPOINT ["/entrypoint.sh"]
 CMD ["caddy", "run", "--config", "/etc/caddy/Caddyfile", "--adapter", "caddyfile"]