Document how to use specific trust server

[jw-s]

Describes how a user can use a different trust server for a repository.

[jw-s]

@liamawhite I obviously don't have access to the IBM internal docs so I can't change that. Do you want to keep the image policy example in the readme?

[molepigeon]

Hi, thanks for the contribution! I've got a couple of comments, mostly to try to keep the style consistent throughout the doc.

[molepigeon]

I obviously don't have access to the IBM internal docs so I can't change that. Do you want to keep the image policy example in the readme?

I'm happy with having an example policy in the README. We don't document the available configuration options, though, so we'll need to either do that or link out to the IBM Cloud docs.

FYI, you can actually submit edits to the IBM Cloud docs through Github - there's an "Edit in Github" link at the top of each topic. The security enforcement ones are here: https://github.com/IBM-Bluemix-Docs/Registry/blob/master/registry_security_enforce.md.

That said, we manage the rollout of the IBM built Container Image Security Enforcement tool separately from Portieris, since it's shipped as a built chart in a repo rather than installed from Github. If you do open a PR against the docs repo, please could you @ me in it so that I can coordinate with our doc writers and make sure the docs accurately reflect what's available in our chart?