Add some end to end test that actually hits the S3 upload code

[rbino]

We already hit a couple of bugs in the upload part of things because S3 upload is stubbed out in tests.

Given that being able to upload a base image is an important piece of functionality that we don't want to regress, we should exercise the actual S3 upload implementation at least with some smoke tests and some local supporting infrastructure (e.g. Minio).