Create CODEOWNERS and validation

[ellahathaway]

Closes #1148

This PR assigns an owner to each dockerfile in the repo & adds PR validation to check that each dockerfile has an owner. The idea is to ping the owner of the file (if a team) or assign the owner to the issue (if an individual).

I did assignments based on git history and the information I received from asking folks which files they own. I had to guess on the ownership of some of the files based on the git history.

We may need to assign write access to the repo for those in the codeowners file with red/error lines.

Here is a link to a sample PR where I tested the validation. This is an example of passing validation and this is an example of failing validation.

NOTE: I don't see the "Actions" tab in this repo, so we may have to enable it for this repo: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository

[ellahathaway]

• Test with failing validation: https://github.com/ellahathaway/dotnet-buildtools-prereqs-docker/actions/runs/10257525841/job/28378724305
• Test with succeeding validation: https://github.com/ellahathaway/dotnet-buildtools-prereqs-docker/actions/runs/10257514236/job/28378689757

[ellahathaway]

The build failure seems unrelated - filed #1160 in response.

[MichaelSimons]

@ellahathaway - would you mind updating the How to modify or create a new image section of the readme to explain that a team code owner must be defined for each Dockerfile and what the responsibilities are? TIA

[ellahathaway]

would you mind updating the How to modify or create a new image section of the readme to explain that a team code owner must be defined for each Dockerfile and what the responsibilities are? TIA

Addressed in #1161