Enable using docker build `--platform` switch (easily)

This is a specific aspect of: https://github.com/dotnet/dotnet-docker/issues/4388

We constructed a “hero Dockerfile” that we want to offer users, but it doesn’t work, sadly. There are multiple points of incompatibility that we need to resolve. See the other issue for more context.

For the SDK/CLI, it comes down to Docker’s and .NET’s syntax/symbols for architecture not matching.

Docker archs:

• amd64
• arm64
• arm

.NET RID archs:

• x64
• arm64
• arm

The archs mostly match across the two systems, except for the first one. We need the RID in -r linux/am64 to be translated to linux/x64. No other change is needed (at least that we’ve discovered).

We only need this for Linux, for two reasons:

• The ENVs we are using are only offered via Linux and buildkit. Windows doesn’t use buildkit.
• Cross-architecture docker isn’t yet a scenario for Windows containers.

[baronfel]

I think the amount of work difference for -r and -a is pretty negligible, so we should go ahead and do both. There’s already an UX inconsistency between CLI options and MSBuild Properties for RuntimeIdentifier so I’d prefer to make sure the CLI options behave as uniformly as possible.

[richlander]

I was just playing with this and realized that -a doesn’t work with dotnet restore.

I’d love for -a to be our guidance for Dockerfiles, however, we’d need that to work for dotnet restore.