Fetch packages for Docker build from s3
Discussed in !1610 (comment 31956692)
As a technical debt cleanup issue, we should just decide to either use ci artifacts, or s3 directly, and then just use that storage api, regardless of triggered/etc. I don't think we need to maintain two codepaths for the storage, at least not for the docker part of things. (I think we have to go to s3 either way in the end for packagecloud)