Optimize CI builds / Speed up integration tests

[jonastheis]

• Execute "Pull additional images" only when they are really needed
  • drand is only needed for dRNG integration test
  • pumba and iproute is only needed for partitions with autopeering
• for building GoShimmer image and executing the tester image: leverage build cache per branch so that subsequent builds on the same branch can reuse Docker build cache and Go build cache (not redownload all modules and no rebuilding of image layers)
• evaluate whether alpine or slim image can be used for tester and whether building an image is better instead of just using one in the docker compose file
• enable cache for unit tests

[jonastheis]

• docker/setup-buildx-action to set up buildkit with install option and limit parallelism
• use actions/cache@v2 to enable caching of layers on Github and configure properly (e.g. hash go.mod) so that cache can be reused, also define restore-keys to fallback to other available caches on the same branch and eventually parent branch https://docs.github.com/en/actions/guides/caching-dependencies-to-speed-up-workflows
• use docker/build-push-action@v2 (https://github.com/docker/build-push-action) to build the image and configure the cache to be used.

Some more resources to study:

• https://evilmartians.com/chronicles/build-images-on-github-actions-with-docker-layer-caching
• https://docs.docker.com/ci-cd/github-actions/
• https://github.com/moby/buildkit#github-actions-cache-experimental

Since we use the Go cache within the Docker image with --mount=type=cache,target=/root/.cache/go-build I'm not entirely sure whether we also need to mount the Go paths: https://github.com/mvdan/github-actions-golang#how-do-i-set-up-caching-between-builds