fix(docker): skip model downloads in CI to prevent HuggingFace CDN 504s

The Docker Build Test was consistently failing because HuggingFace CDN
returns 504 Gateway Timeout when downloading the LaMa ONNX model (~200MB)
from GitHub Actions runners. Model availability is an external dependency,
not something CI can control.

Added SKIP_MODEL_DOWNLOADS build arg (default: false). When set to true,
the download_models.py step is skipped entirely. CI only needs to verify
the image structure builds — Python deps install, Node build runs, app
code is copied — not that every ML model CDN is reachable.

Production builds (docker build without the arg) still download all models
as before.

.github/workflows/ci.yml

@@ -93,5 +93,6 @@ jobs:
           file: docker/Dockerfile
           push: false
           tags: ashim:ci
+          build-args: SKIP_MODEL_DOWNLOADS=true
           cache-from: type=gha,scope=unified
           cache-to: type=gha,mode=max,scope=unified