build: optimize Docker setup for Caddy SPA and improve runtime stability

Updated Dockerfile to streamline the hybrid build process and optimize artifact integration. Modified Caddyfile for better routing logic, enhanced security headers, Prometheus metrics, and improved API proxy handling. Adjusted `dc-gui.yaml` for container stability and simplified runtime configuration for `apiBaseUrl`. Expanded documentation with troubleshooting and session logs.

dc-gui.yaml

@@ -11,8 +11,6 @@ services:
       context: . # Wichtig: Root Context für Monorepo Zugriff
       dockerfile: config/docker/caddy/web-app/Dockerfile
       args:
-        GRADLE_VERSION: "${DOCKER_GRADLE_VERSION:-9.3.1}"
-        JAVA_VERSION: "${DOCKER_JAVA_VERSION:-25}"
         # Frontend spezifisch:
         CADDY_VERSION: "${DOCKER_CADDY_VERSION:-2.9-alpine}"
         # Metadaten:
@@ -21,14 +19,12 @@ services:
     labels:
       - "org.opencontainers.image.created=${DOCKER_BUILD_DATE}"
     container_name: "${PROJECT_NAME:-meldestelle}-web-app"
-    restart: no
+    restart: unless-stopped
     ports:
       - "${WEB_APP_PORT:-4000:4000}"
     environment:
       # Runtime Configuration for Caddy Templates
       # Browser can access API via localhost:8081 (Gateway)
-      # In Docker network, it might be http://api-gateway:8081, but browser runs on host!
-      # Usually, for local dev, we want the browser to hit localhost:8081.
       API_BASE_URL: "${WEB_APP_API_URL:-http://localhost:8081}"
     depends_on:
       api-gateway: