fix: resolve ENOSPC on CI by using BuildKit pip cache and registry layer cache

- Add `# syntax=docker/dockerfile:1` to enable BuildKit cache mounts
- Replace `--no-cache-dir` pip installs with `--mount=type=cache,target=/root/.cache/pip`
  so torch/CUDA wheels are reused across builds instead of re-downloaded every run
- Add `docker system prune -f` step before build to free dangling image/layer space
- Add `cache-from`/`cache-to` pointing to `:cache` tag so unchanged layers
  (including the heavy voice-deps layer) are pulled from registry instead of rebuilt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # Stage 1: Build Vue frontend
 FROM node:22-alpine AS build-frontend
 WORKDIR /build
@@ -12,16 +13,20 @@ WORKDIR /app
 
 COPY pyproject.toml .
 COPY src/ src/
-RUN pip install --no-cache-dir .
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install .
 # Voice dependencies (faster-whisper, Kokoro TTS, soundfile) — activated at runtime via VOICE_ENABLED
-RUN pip install --no-cache-dir faster-whisper kokoro soundfile
+# Cache mount keeps wheels across builds so the heavy torch/CUDA deps aren't re-downloaded every time.
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install faster-whisper kokoro soundfile
 
 # Build the fable-mcp wheel so it can be served for download
 COPY fable-mcp/ fable-mcp/
-RUN pip install --no-cache-dir build hatchling \
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install build hatchling \
     && python -m build --wheel ./fable-mcp --outdir /app/dist/ \
     && pip uninstall -y build \
-    && rm -rf fable-mcp/ /root/.cache/pip
+    && rm -rf fable-mcp/
 
 COPY --from=build-frontend /build/dist/ src/fabledassistant/static/
 COPY alembic.ini .