Fix Docker build: add .dockerignore, fix permissions

- Add .dockerignore to exclude instance/, test_data/, rpi/, etc.
- Create instance/certs dirs in Dockerfile for volume mounts
- Ensures stego user can write to mounted volumes

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -62,8 +62,8 @@ COPY src/ src/
 COPY data/ data/
 COPY frontends/web/ frontends/web/
 
-# Create upload directory
-RUN mkdir -p /tmp/stego_uploads
+# Create upload directory and instance directories (for volumes)
+RUN mkdir -p /tmp/stego_uploads /app/frontends/web/instance /app/frontends/web/certs
 
 # Create non-root user
 RUN useradd -m -u 1000 stego && chown -R stego:stego /app /tmp/stego_uploads