feat: add Docker image build jobs to CI workflow (Closes #2)

[ZacLou]

Closes #2

Changes

Adds a docker job to the CI workflow that builds both Docker images on every PR and push to main:

• docker job: Depends on rust job passing first, runs on ubuntu-latest
• Uses docker/setup-buildx-action@v3 for Buildx setup
• API image: Builds Dockerfile.api with GitHub Actions cache
• Gateway image: Builds Dockerfile.gateway with GitHub Actions cache
• push: false — images are built but not pushed (no registry configured)
• cache-from/to: type=gha — uses GitHub Actions cache for faster rebuilds

Validation

• Docker builds run in CI without pushing, catching Dockerfile issues early
• Cache reduces rebuild time for unchanged layers