instruction: Explain the POST /projects/:id/pipeline Attribute Type Required Description id integer/string Yes The ID or URL-... in gitlab_api
output: POST /projects/:id/pipeline Attribute Type Required Description id integer/string Yes The ID or URL-encoded path of the project ref string Yes The branch or tag to run the pipeline on. For merge request pipelines use the merge requests endpoint . variables array No An array of hashes containing the variables available in the pipeline, matching the structure [{ 'key': 'UPLOAD_TO_S3', 'variable_type': 'file', 'value': 'true' }, {'key': 'TEST', 'value': 'test variable'}] . If variable_type is excluded, it defaults to env_var . in