ci: add workflow to build and push Docker image

Problem

There is no CI workflow that builds or pushes the livepeer/subgraph Docker image. The image doesn't exist on Docker Hub, and the Docker-based test infrastructure (docker-compose.test.yml) has been broken without anyone noticing since CI never exercises it.

Currently all Docker builds are manual, and the test script (yarn test) relies on a Docker image that is never built or published by CI.

Plan

1. First: Fix the docker-compose v1 issues (#220) and create a PR
   • Replace docker-compose with docker compose in package.json scripts
   • Remove --cache-from for non-existent image in docker:build
   • Remove deprecated version: "3" from compose files
   • Update stale container images (ipfs/go-ipfs, etc.)
2. Then: Add a GitHub Actions workflow that:
   • Builds the livepeer/subgraph Docker image on PRs (smoke test)
   • Pushes the image to a container registry (Docker Hub or GHCR) on merges to main and version tags
   • Optionally runs the docker-compose.test.yml integration test suite as part of CI

This would catch issues like the recent node:14 / node:20 Dockerfile breakage (#219) before they land.

Related

• #220 — docker-compose v1 dependency breaks test and start scripts

🤖 Generated with Claude Code

Start with the package.json scripts, docker-compose.test.yml, and Dockerfile, and review issue #220 before designing the workflow. Verify that the Docker image builds on pull requests, that merges to main and version tags publish it to the selected registry, and that the Docker-based tests are exercised in CI.