Fix: Isolated workflow code execution in a Docker container.

[BroehainIsHere]

Fixes #215. Given the lack of a discussion branch, I've submitted the PR to main.

This PR addresses the issue discussed in #215.

• All workflow code is now executed in an isolated Docker container;
• User code receives input via /app/input.json and must export an async code function;
• Output and errors are captured and returned as structured JSON;
• The container runs as a non-root user, with no network and strict resource limits;
• A feature flag allows fallback to in-process execution for debugging.

This patch prevents cross-user code/data access and closes the RCE vector.

[Lerenah3]

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[Lerenah3]

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[Lerenah3]

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