build(docker): slim backend image, static frontend image, -docling variant

Backend (arc53/docsgpt): 4.5 GB compressed -> 0.9 GB with both embedding
models and tiktoken baked in.
- torch/transformers gone from the default install (docling extra only).
- Ubuntu 24.04 ships python3.12: no deadsnakes PPA, no software-properties-
  common; every pin is a wheel, so no gcc/g++/rust in the builder.
- COPY --chown and a prefetch that runs as the process user replace the
  trailing chown -R, which duplicated the 600 MB model layer.
- .dockerignore keeps __pycache__, .coverage, local indexes and .env out.
- EXTRAS build arg (INSTALL_DOCLING kept as an alias); the docling variant
  also bakes docling's layout/table/RapidOCR models (DOCLING_ARTIFACTS_PATH)
  and tesseract, and drops only the discovery documents of Google APIs the
  app never builds.
- FLASK_DEBUG env removed (unused); OCI labels added.

Frontend (arc53/docsgpt-fe): 302 MB Vite dev server -> 25 MB static build
behind nginx. VITE_* variables are injected at container start into
/config.js and read through src/env.ts, so the image no longer needs a
rebuild per deployment; docker-compose.yaml keeps hot reload via the dev
target.

Publishing: every release and develop build now pushes a slim tag and a
-docling tag (docling engine + models + tesseract). docker-compose-hub.yaml
takes DOCSGPT_IMAGE_TAG / DOCSGPT_IMAGE_VARIANT; docker-compose-standalone.yaml
runs the stack from pre-built images without a checkout and is attached to
each release. setup.sh selects the -docling variant for OCR instead of
requiring a local build. A new workflow builds the image on PRs that touch
it and runs verify_offline under --network none; lint checks the exported
requirements match uv.lock.

.github/workflows/lint.yml

@@ -20,3 +20,18 @@ jobs:
         uses: chartboost/ruff-action@v1
         with:
           version: 0.14.10
+
+  requirements-in-sync:
+    # application/requirements*.txt are exported from uv.lock; fail when a
+    # change to pyproject.toml or uv.lock was not re-exported.
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: astral-sh/setup-uv@v6
+
+      - name: Re-export and diff
+        run: |
+          uv lock --check
+          bash scripts/export_requirements.sh
+          git diff --exit-code -- application/requirements.txt application/requirements-docling.txt application/requirements-milvus.txt