Ensure all Dockerfiles reside within an <architecture> directory

[MichaelSimons]

Recently there have been some PRs that started to introduce ARM Dockerfiles. Previously all Dockerfiles were AMD64 based. As a result of this, the guidance was updated to place the Dockerfiles in an architecture specific directory. This is the convention previously used in https://github.com/dotnet/dotnet-docker.

The existing Dockerfiles/directory structure should be updated accordingly to follow this convention.