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/docker-image-verify.yml

@@ -0,0 +1,52 @@
+name: Verify the Docker image works offline
+
+# Builds the backend image and runs its offline check with networking off, so
+# a change that reintroduces a first-request download (a tokenizer, tiktoken's
+# encoding, an embedding model) fails here instead of in an air-gapped install.
+
+on:
+  workflow_dispatch:
+  pull_request:
+    paths:
+      - 'application/Dockerfile'
+      - 'application/.dockerignore'
+      - 'application/requirements*.txt'
+      - 'application/scripts/prefetch_models.py'
+      - 'application/scripts/verify_offline.py'
+      - 'application/vectorstore/model_registry.py'
+      - 'application/parser/tokenization.py'
+      - 'application/vectorstore/embeddings_local.py'
+      - '.github/workflows/docker-image-verify.yml'
+
+permissions:
+  contents: read
+
+jobs:
+  verify:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build the slim image
+        uses: docker/build-push-action@v6
+        with:
+          file: ./application/Dockerfile
+          context: ./application
+          platforms: linux/amd64
+          load: true
+          tags: docsgpt:verify
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - name: Image size
+        run: |
+          docker image inspect docsgpt:verify --format '{{.Size}}' | awk '{printf "uncompressed: %.2f GB\n", $1/1e9}'
+          docker history docsgpt:verify --format '{{.Size}}\t{{.CreatedBy}}' | head -20
+
+      - name: Offline verification (no network)
+        run: |
+          docker run --rm --network none docsgpt:verify \
+            python -m application.scripts.verify_offline