feat(indexer): Dockerfile for indexer crate

[Depo-dev]

Summary

No Dockerfile exists for the indexer. The production docker-compose.yml references a build context at crates/indexer but there is no Dockerfile there.

Acceptance Criteria

• Multi-stage Dockerfile: builder stage (rust:1.78-slim) + runtime stage (debian:bookworm-slim)
• Builder: cargo build --release -p trident-indexer
• Runtime: copy binary only, no Rust toolchain in final image
• Final image < 100 MB
• docker build succeeds locally and in CI
• Add a docker-build job to .github/workflows/ci.yml that builds (but does not push) both images

Files

crates/indexer/Dockerfile (new), .github/workflows/ci.yml