feat: Docker quickstart with Compose, docs, and improved Dockerfile

Rewrites Dockerfile to use bookworm-slim base, installs Claude and Codex
CLIs, adds docker-compose.quickstart.yml for one-command setup, and adds
DOCKER.md with usage instructions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,10 @@
+.git
+.github
+.paperclip
+.pnpm-store
+node_modules
+**/node_modules
+coverage
+data
+tmp
+*.log