feat: add junie Dockerfile for Docker image builds (#2601)

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>

Author: 3 people

sh/docker/junie.Dockerfile

@@ -0,0 +1,17 @@
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Base packages
+RUN apt-get update -y && \
+    apt-get install -y --no-install-recommends \
+      curl git ca-certificates build-essential unzip zsh && \
+    rm -rf /var/lib/apt/lists/*
+
+# Node.js 22 via n
+RUN curl --proto '=https' -fsSL https://raw.githubusercontent.com/tj/n/master/bin/n | bash -s install 22
+
+# Junie CLI
+RUN npm install -g @jetbrains/junie-cli
+
+CMD ["/bin/sleep", "inf"]