Add missed .env file for React

[bidzhao]

Description

Add missed .env file for React.

Fixes # (issue)

Specify build target path, to solve "failed to compute cache key: "/ui/dist" not found: not found".

Type of change

(Delete what does not apply)

• Bug fix (non-breaking change which fixes an issue)

Checklist

(Delete what does not apply)

• [Y] Have you checked to there aren't other open Pull Requests for the same update/change?
• [Y] Have you lint your code locally before submission?
• [Y] Have you added an explanation of what your changes do and why you'd like us to include them?
• [N] Have you written new tests for your core changes, as applicable?
• [Y] Have you successfully run tests with your changes locally?