Enhance Nexus CI test with docker registry push/pull

Since we now have jobs that can create repositories we should enhance the CI tests to perform more than a simple login.

One option would be to create a docker registry in nexus, then validate pushing/pulling to/from it.

In BB you can pass these values to nexus to setup a container registry (nest these under addons.nexus):

nexus:
  docker:
    enabled: true
    registries:
      - host: containers.bigbang.dev
        port: 5000
  repository:
    enabled: true
    repo:
      - name: "containers"
        format: "docker"
        type: "hosted"
        repo_data:
          name: "containers"
          online: true
          storage:
            blobStoreName: "default"
            strictContentTypeValidation: true
            writePolicy: "allow_once"
          cleanup:
            policyNames:
              - "string"
          component:
            proprietaryComponents: true
          docker:
            v1Enabled: false
            forceBasicAuth: true
            httpPort: 5000

When testing in the package you may need to modify this slightly since containers.bigbang.dev will not resolve (no Istio in package pipeline).