All 7 other agents have a sh/docker/{agent}.Dockerfile; junie was added
in 2026-03 but its Dockerfile was never created, meaning no Docker image
exists for it. This adds the missing file following the codex pattern
(npm-based agent, Node.js 22 via n).
Note: .github/workflows/docker.yml also needs `junie` added to its
matrix.agent array — tracked in a separate GitHub issue.
Agent: team-lead
Co-authored-by: B <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>
