fix: Ensure proper cache directory ownership in Dockerfile (#46)

## Summary

Fixed agent cache directory ownership in Dockerfile by ensuring the
playwright cache directory is created with proper ownership before
browser installation.

## Changes

- Created cache directory `/home/agent/.cache/ms-playwright` before
playwright installation
- Set correct ownership (`agent:a2a`) for the cache directory before
browser installation
- This prevents permission issues during container runtime

## Benefits

1. **Prevents permission issues**: Agent user has proper write access to
cache directory
2. **Maintains security**: Non-root agent user can properly access
browser binaries
3. **Build consistency**: Docker image builds successfully with correct
ownership structure

The fix ensures that when the agent runs and checks for browser
installation, it won't encounter permission issues accessing the cache
directory at `/home/agent/.cache/ms-playwright`.

Author: edenreich