Multi-select inputs in pipeline UI
**Problem** GitLab CI/CD pipeline inputs with dropdown options currently only allow selecting a single value. Users need the ability to select multiple values from the dropdown options. **Desired Feature** Users want to select multiple options which would be combined into a single string type value using a delimiter. **Use Cases**: * Restarting services on multiple instances * Building multiple Docker images in one pipeline * Running tests with multiple tag combinations * Any scenario requiring operations across multiple targets
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