Add automated test

[manics]

• Setup two registries (one for caching, one for the final image) with credentials
• run repo2docker --engine=kaniko ...
• Check the cache contains something
• Check the final image is pushed to the other registry

e.g. using https://github.com/yodle/docker-registry-client

#!/usr/bin/env python

from docker_registry_client import DockerRegistryClient

REGISTRY = "http://localhost:5000"

client = DockerRegistryClient(REGISTRY)
repositories = client.repositories()
print("Repositories:")
for r in repositories:
    print(f"  {r}")

if "cache" in repositories:
    print("cache tags:")
    cache = client.repository("cache")
    tags = cache.tags()
    cache_manifests = dict((t, cache.manifest(t)) for t in tags)
    for t, manifest in cache_manifests.items():
        print(f"  {t}: {manifest}")