Add automated testing against well known registries on commits and releases

### Acceptance Criteria:
**As a contributor to imgpkg**
I want to have confidence that my changes did not introduce inconsistent behavior on different registries.

**When a new commit is pushed to imgpkg**, I expect to see github actions running integration tests on:
- [x] Harbor
- [x] GCR

**When a new release of imgpkg is cut**, I expect to see github actions running integration tests on:
- [x] Harbor
- [x] GCR
- [ ] https://cloud.google.com/artifact-registry
- [ ] DockerHub

**Notes:**
- Since DockerHub has rate limiting, running the integration tests only during a release is sufficient.
- For the integration with harbor, Joao found that we could stand up an instance using https://github.com/bitnami/bitnami-docker-harbor-portal/blob/master/docker-compose.yml, so maybe we could just start a container to do this.
- **Feel free to tackle this work all at once, or one registry at a time using 3 different PRs.**
- in fact, if supporting GCR and DockerHub is not "just" an incremental investment, consider whether it makes sense to tackle those registries, later.

👇🏻 Original request👇🏻

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- **Describe the problem/challenge you have**
As a user, I want to be sure that `imgpkg` works with different registries

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**Describe the solution you'd like**
When testing `imgpkg` using Github action, we should try to test against a local registry and against more widely spread registries, like gcr, harbor, and dockerhub.

**Anything else you would like to add:**
I spun up this issue from #131, thanks for the idea @DennisDenuto
For this issue, we should do the following:
- [x] Run integration tests against a gcr instance
- [x] Run integration tests against an Harbor instance
- Is there a Harbor instance that we can use?
- Should we instantiate a Harbor instance and use it for testing?
- Can we run Harbor as a container similar to what we are doing with the registry:2?
- [ ] Run integration tests against a dockerhub, we should be careful about this due to rate limiting. Maybe we could consider running these tests as part of a Release process

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