Removed custom Dockerfile and switched to using the official
machines/filestash image from Docker Hub:
- Removed stash/Dockerfile (custom build with ffmpeg)
- Updated compose.yaml to use ${STASH_IMAGE} variable
- Defaults to machines/filestash:latest
This simplifies maintenance and ensures we use the officially
maintained image with proper updates via Watchtower.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>