Currently auth.docker.io uses a custom auth scope for (docker) plugins
`repository(plugin):<repo>:<perms>`.
This makes it impossible to use containerd distribution tooling to fetch
plugins without also supplying a totally custom authorizer.
This changes allows clients to set the correct scope on the context.
It's a little bit nasty but "works".
I'm also a bit suspect of some a couple of these unexported context
functrions. Before the primary one used `contextWithRepositoryScope`
overwrites any scope value and there is another one that appends the
scope value.
With this change they both append...
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
e84a84a5a9