fix: resolve container startup issues

backend/Dockerfile:
- Add ENV PYTHONPATH=/app so alembic can import app.config
- Change CMD to bash entrypoint.sh (volume mount breaks chmod +x)

nginx/nginx.conf:
- Add resolver 127.0.0.11 (Docker internal DNS)
- Use set $backend/$frontend variables so nginx resolves
  hostnames per-request instead of at startup

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -6,6 +6,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends postgresql-clie
 
 WORKDIR /app
 
+ENV PYTHONPATH=/app
+
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
@@ -13,4 +15,4 @@ COPY . .
 
 RUN chmod +x entrypoint.sh
 
-CMD ["./entrypoint.sh"]
+CMD ["bash", "entrypoint.sh"]