A branch such as feature/foo produced an invalid Docker tag, so the build aborted before it started. Replace invalid characters with a dash, strip leading dots and dashes, and cap the length at 128. master still maps to latest. The branch name now reaches the shell through an env var, because a name containing $(...) was previously executed by the step. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>