feat: serve the UI from the backend image, one-port standalone stack

The backend image builds the web UI with scripts/build_frontend.sh and
serves it through docsgpt/ui.py, so the standalone Compose file drops the
frontend container. UI and API share port 7091, published on 127.0.0.1
unless DOCSGPT_BIND says otherwise. POSTGRES_PASSWORD is configurable, and
an optional https profile puts Caddy in front of a public domain.

docker-image-verify.yml starts the standalone stack on the image it built
and checks the API, the UI, /config.js and a client-side route on one port.

docsgpt/Dockerfile

@@ -1,4 +1,4 @@
-# DocsGPT backend image.
+# DocsGPT image: the API, the web UI it serves, and the Celery worker.
 #
 # Build args:
 #   EXTRAS              comma-separated optional extras to bake in, matching the
@@ -15,6 +15,19 @@
 # docling's layout/table/OCR models. `python -m docsgpt.scripts.verify_offline`
 # under `docker run --network none` proves it.
 
+# The web UI, built by the same script the Python package build runs. The API
+# serves it from docsgpt/static (docsgpt/ui.py). The output is static files, so
+# this stage runs on the build machine's platform whatever the target is.
+FROM --platform=$BUILDPLATFORM node:22-bookworm-slim AS ui
+
+WORKDIR /src
+COPY frontend/package.json frontend/package-lock.json frontend/
+RUN cd frontend && npm ci --include=dev --no-audit --no-fund
+COPY frontend/ frontend/
+COPY scripts/build_frontend.sh scripts/
+RUN mkdir docsgpt && bash scripts/build_frontend.sh
+
+
 FROM ubuntu:24.04 AS builder
 
 ENV DEBIAN_FRONTEND=noninteractive
@@ -87,7 +100,7 @@ RUN if [ "$INSTALL_TESSERACT" = "true" ]; then \
 
 LABEL org.opencontainers.image.source="https://github.com/arc53/DocsGPT" \
       org.opencontainers.image.title="DocsGPT" \
-      org.opencontainers.image.description="DocsGPT backend: API and Celery worker" \
+      org.opencontainers.image.description="DocsGPT: API, web UI and Celery worker" \
       org.opencontainers.image.licenses="MIT"
 
 WORKDIR /app
@@ -136,6 +149,7 @@ RUN if python -c "import docling" 2>/dev/null; then \
     fi
 
 COPY --chown=appuser:appuser docsgpt /app/docsgpt
+COPY --from=ui --chown=appuser:appuser /src/docsgpt/static /app/docsgpt/static
 # One-release alias so `-A application.app.celery` style entry points keep working.
 COPY --chown=appuser:appuser application/__init__.py /app/application/__init__.py