Create an ongoing changelog/"breaking change" document in eng/docker-tools

[lbussell]

Problems

1. It's not often easy to tell if a docker-tools update PR contains breaking changes
2. If a PR contains breaking changes, it's not usually obvious how to react to the change.

Proposed solution

An ongoing changelog in eng/docker-tools/changelog.md.

• The changelog should be manually updated whenever there are new features or breaking changes.
• The changelog should be updated in the same PR that the breaking change is introduced in.
• Each change in the changelog should link to the pull request (and optionally the issue) for the change.

When consuming repos receive new changes, they will only need to look at the changelog to know whether there are new features/breaking changes that they need to react to, without reading any of the changes in scripts or pipeline templates.

[lbussell]

We should start this with ACR authentication can fail when using two different service connections for the same ACR (dotnet/docker-tools#1914). The fix (#1945) requires a breaking change to the PublishConfiguration model.