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>

doc/DOCKER.md

@@ -0,0 +1,68 @@
+# Docker Quickstart
+
+Run Paperclip in Docker without installing Node or pnpm locally.
+
+## One-liner (build + run)
+
+```sh
+docker build -t paperclip-local . && \
+docker run --name paperclip \
+  -p 3100:3100 \
+  -e HOST=0.0.0.0 \
+  -e PAPERCLIP_HOME=/paperclip \
+  -v "$(pwd)/data/docker-paperclip:/paperclip" \
+  paperclip-local
+```
+
+Open: `http://localhost:3100`
+
+Data persistence:
+
+- Embedded PostgreSQL data
+- uploaded assets
+- local secrets key
+- local agent workspace data
+
+All persisted under your bind mount (`./data/docker-paperclip` in the example above).
+
+## Compose Quickstart
+
+```sh
+docker compose -f docker-compose.quickstart.yml up --build
+```
+
+Defaults:
+
+- host port: `3100`
+- persistent data dir: `./data/docker-paperclip`
+
+Optional overrides:
+
+```sh
+PAPERCLIP_PORT=3200 PAPERCLIP_DATA_DIR=./data/pc docker compose -f docker-compose.quickstart.yml up --build
+```
+
+## Claude + Codex Local Adapters in Docker
+
+The image pre-installs:
+
+- `claude` (Anthropic Claude Code CLI)
+- `codex` (OpenAI Codex CLI)
+
+If you want local adapter runs inside the container, pass API keys when starting the container:
+
+```sh
+docker run --name paperclip \
+  -p 3100:3100 \
+  -e HOST=0.0.0.0 \
+  -e PAPERCLIP_HOME=/paperclip \
+  -e OPENAI_API_KEY=... \
+  -e ANTHROPIC_API_KEY=... \
+  -v "$(pwd)/data/docker-paperclip:/paperclip" \
+  paperclip-local
+```
+
+Notes:
+
+- Without API keys, the app still runs normally.
+- Adapter environment checks in Paperclip will surface missing auth/CLI prerequisites.