Three issues prevented the Docker terminal backend from working:
1. `effective_image` was referenced but never defined — only the Modal
backend sets this variable. Use `image` directly instead.
2. `--storage-opt size=N` is unsupported on Docker Desktop for Mac
(requires overlay2 with xfs backing). Skip the flag on Darwin.
3. Docker requires absolute paths for `-w` (working directory) but the
default cwd was `~`, which Docker does not expand. Default to `/root`
and translate any `~` passed in from callers.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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