Add registry layer cache and fix Dockerfile cache order

- CI: cache-from/cache-to with mode=max stores all intermediate layers
  in the registry so subsequent builds skip unchanged layers (especially
  the slow ROCm PyTorch download)
- Dockerfile: move COPY perth_stub.py below pip install layers so a
  stub change doesn't bust the cache for everything above it

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

.gitea/workflows/build.yml

@@ -31,3 +31,5 @@ jobs:
           tags: |
             git.sdgarren.com/scott/rocm-chatterbox-whisper:latest
             git.sdgarren.com/scott/rocm-chatterbox-whisper:${{ gitea.sha }}
+          cache-from: type=registry,ref=git.sdgarren.com/scott/rocm-chatterbox-whisper:buildcache
+          cache-to: type=registry,ref=git.sdgarren.com/scott/rocm-chatterbox-whisper:buildcache,mode=max

Dockerfile.rocm

@@ -17,8 +17,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 
 WORKDIR /app
 
-COPY perth_stub.py .
-
 # Step 1: Install ROCm-compatible PyTorch stack first.
 # This must happen before anything else to prevent pip from pulling CPU wheels.
 COPY requirements-rocm-init.txt .
@@ -32,7 +30,8 @@ RUN pip3 install -r requirements-rocm.txt
 RUN pip3 install --no-deps chatterbox-tts
 
 # Stub out resemble-perth (audio watermarking, unnecessary for self-hosted use).
-# Install as 'perth' module to satisfy chatterbox's import.
+# Placed after pip layers so changes to the stub don't bust the cache above.
+COPY perth_stub.py .
 RUN python3 -c "import site; print(site.getsitepackages()[0])" | \
     xargs -I{} cp /app/perth_stub.py {}/perth.py