feat: Dockerfile for containerized deployments

[dobexx]

What

Adds a production-ready Dockerfile (+ .dockerignore) so the proxy can run as a container service (Docker Compose, EasyPanel, Kubernetes, ...).

Design:

• Multi-stage build on node:22-slim: builder compiles TypeScript, runtime ships only production deps
• Claude Code CLI preinstalled globally (the proxy wraps it as a subprocess)
• Non-root user proxyapp (node:*-slim base images already ship a system user named proxy, hence the distinct name)
• Credentials persistence: CLAUDE_CONFIG_DIR=/data/.claude – mounting a volume on /data keeps OAuth tokens across restarts/redeploys (the CLI refreshes tokens in place)
• Healthcheck against /health
• HOST=0.0.0.0 / PORT=3456 presets for container networking

Context: PR #9 (Docker CI workflow) was closed without merge – this PR is different in scope: it ships the Dockerfile itself, no CI changes.

Tested: Running in production on an EasyPanel (Docker Swarm) host – build, healthchecks, volume persistence across redeploys all verified live.

Note: pairs naturally with the auth/.env PR I'll open next (the proxy currently has no request authentication, which matters once it listens on a routable address).

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider pinning the Node base image and the @anthropic-ai/claude-code dependency to specific versions (or using build args) to avoid unexpected breakage from upstream changes.
• The .dockerignore file is currently empty; add common exclusions like node_modules, dist, .git, and local config files to minimize build context size and speed up builds.
• The healthcheck relies on an inline node -e script; you might simplify and harden this by using a small shell-based check (e.g., wget/curl if available) or a dedicated healthcheck script in the repo for easier maintenance.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider pinning the Node base image and the `@anthropic-ai/claude-code` dependency to specific versions (or using build args) to avoid unexpected breakage from upstream changes.
- The `.dockerignore` file is currently empty; add common exclusions like `node_modules`, `dist`, `.git`, and local config files to minimize build context size and speed up builds.
- The healthcheck relies on an inline `node -e` script; you might simplify and harden this by using a small shell-based check (e.g., `wget`/`curl` if available) or a dedicated healthcheck script in the repo for easier maintenance.

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[dobexx]

Thanks for the review! Notes on the two points:

• Version pinning: deliberately kept node:22-slim (follows the LTS line) and unpinned @anthropic-ai/claude-code so redeploys pick up current CLI/model support automatically. Happy to pin to a digest if you prefer reproducibility over freshness – just say the word.
• .dockerignore: the file is not empty – it excludes node_modules, dist, .git, .env, docs etc. (possibly it wasn't readable in the review context).

@sourcery-ai review