feat: support external Docker database seed

[thiagozs]

Summary

• download and extract an external BadgerDB seed from DATA_DNE_EXTERNAL when the configured database is absent
• persist database files safely in named volumes while running the API as a non-root user
• improve Docker build caching, health checks, runtime logging, and configurable image publishing
• add a Portainer/Swarm stack example and update Docker documentation

Validation

• go test ./...
• sh -n docker-entrypoint.sh
• docker compose config
• docker compose -f docker-compose.portainer.yml config
• complete Docker image build
• runtime test with an empty named volume and external seed (1,610,653 CEPs loaded)