fix: address code review issues with Docker setup

- .gitignore: whitelist Dockerfile, docker-compose.yml, docker/ so they
  are actually tracked (the * catch-all was silently dropping them)
- Dockerfile: build and ship the archivr CLI alongside archivr-server so
  users can run `archivr init` inside the container on first setup
- docker/config.example.toml: fix archive_path to point at the .archivr
  subdirectory that archivr init creates (not the parent directory), which
  is what read_archive_paths expects
- docs/README.md: replace the bare mkdir quickstart step with
  `archivr init`, explain why mkdir is insufficient; add a callout that
  auth_db_path must be set explicitly to a writable path when the config
  mount is read-only

docker-compose.yml

@@ -0,0 +1,23 @@
+services:
+  archivr:
+    build: .
+    image: archivr-server:latest
+    restart: unless-stopped
+    ports:
+      - "8080:8080"
+    environment:
+      # Listen on all interfaces inside the container so the port mapping works.
+      ARCHIVR_BIND: "0.0.0.0:8080"
+      # Uncomment and set this to enable Twitter/X archiving.
+      # The file must be accessible inside the container (e.g. in the config volume).
+      # ARCHIVR_TWITTER_CREDENTIALS_FILE: /config/twitter-cookies.txt
+    volumes:
+      # Mount a directory containing archivr-server.toml as read-only config.
+      # Copy docker/config.example.toml to ./config/archivr-server.toml to start.
+      - ./config:/config:ro
+      # Persistent volume for the auth database and archive directories.
+      # The paths inside must match archive_path values in your TOML config.
+      - archivr-data:/data
+
+volumes:
+  archivr-data: