Add Docker setup for BitcoinPurple explorer

- Dockerfile: node:20-alpine, compiles SCSS at build time, single image
  used for web server and all sync modes (blocks, peers, markets)
- docker-compose.yml: explorer + MongoDB (bind-mounted ./db) on shared
  'purple' network alongside bitcoinpurpled and electrumx
- docker/entrypoint.sh: generates settings.json from env vars via
  envsubst, dispatches to web/sync-blocks/sync-peers/reindex modes
- docker/settings.json.tmpl: minimal settings template parametrized
  for BitcoinPurple (coin, wallet RPC, MongoDB, theme)
- docker/mongo-init.sh: creates app user in explorerdb on first start
- .env.example: pre-filled defaults for BitcoinPurple
- CLAUDE.md: codebase guidance for Claude Code
- .gitignore: add db/ (MongoDB bind-mount data directory)

.dockerignore

@@ -0,0 +1,12 @@
+node_modules/
+db/
+tmp/
+.git/
+.gitignore
+settings.json
+*.log
+explorerdb_dump.7z
+docker-compose.yml
+.env
+.env.*
+!.env.example