Define owner(s) for each Dockerfile

[MichaelSimons]

There is a need to identify the owner(s) of the build tools Dockerfiles. The owner will be contacted/assigned issues in the following scenarios:

1. When there is a build break.
2. When there is a fixable vulnerability that requires a code change (e.g. a simple rebuild will not address).

Considerations:

1. Where to define the owner? CODEOWNERS feels like a natural mechanism. The manifest.json could be modified to support this as well.
2. Are teams allowed? Issues cannot be assigned to an issue. You could assign issues to all members of a team but that doesn't provide a great dev UX.
3. Validation should be created to ensure all Dockerfiles have an owner defined.

Related to https://github.com/dotnet/dotnet-buildtools-prereqs-docker/issues/972