Support authentication for FROM image references.

[qu1queee]

In today's community meeting, we discussed about lack of support for the authentication on the FROM image references in Dockerfile's . This was brought today by two different people.

We have an existing issue that dates back to some years, where another community member brought similar points on this.

With recent announcements such as DockerHub reducing rate limits for anonymous pulls again from 100 pulls per six hours per IP to 10 pulls per one hour per IP. It makes sense to bring this item to this community attention and decided on next steps.

[SaschaSchwarze0]

API wise, I suggest to have the following:

kind: Build
spec:
  pullSecrets:
  - secret-1
  - secret-2

Runtime-wise, we can today simply add them to the generated service account and Tekton's creds-init handles it. Though, it is maybe also time to finally get rid of it which means that we must implement our own small binary and a new step that merges multiple secrets into a single one and stores that in an emptyDir with memory storage. This would only be needed in case there are multiple secrets (pullSecrets + pushSecret matter).

I would be stricter than Tekton when it comes to overlapping host names across multiple secrets and would fail the BuildRun if that is the case. I think that Tekton uses the first or last one when there are overlaps.

TBD: for the image processing step, I guess we continue to only use the pushSecret.