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.

deployment/docker-compose.yaml

@@ -1,7 +1,11 @@
 name: docsgpt-oss
 services:
   frontend:
-    build: ../frontend
+    build:
+      context: ../frontend
+      # Vite dev server with hot reload over the bind mount below. The default
+      # target (what Docker Hub publishes) is a static build behind nginx.
+      target: dev
     volumes:
       - ../frontend/src:/app/src
     environment:
@@ -18,8 +22,10 @@ services:
     build:
       context: ../application
       args:
-        # Bake the optional docling engine (layout-model OCR backend, structured
-        # output) into the image: set INSTALL_DOCLING=true in ../.env or the shell.
+        # Optional extras to bake in (comma-separated): docling, milvus. The
+        # docling extra brings the layout-model parser/OCR backend and its
+        # models. INSTALL_DOCLING=true is the older spelling of EXTRAS=docling.
+        EXTRAS: ${EXTRAS:-}
         INSTALL_DOCLING: ${INSTALL_DOCLING:-false}
         # Bake the tesseract binary behind OCR_ENABLED=true (~35 MB): set
         # INSTALL_TESSERACT=true in ../.env or the shell (setup.sh does this
@@ -53,6 +59,7 @@ services:
     build:
       context: ../application
       args:
+        EXTRAS: ${EXTRAS:-}
         INSTALL_DOCLING: ${INSTALL_DOCLING:-false}
         INSTALL_TESSERACT: ${INSTALL_TESSERACT:-false}
     # Consumes the default queue AND the dedicated `parsing` (read_document /