Enable using docker build --platform switch (easily)

[richlander]

This is a specific aspect of: dotnet/dotnet-docker#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]

When adding this mapping, we should cover -r/--runtime, and -a/--arch, but not values for RuntimeIdentifier passed explicitly via MSBuild properties (-p RuntimeIdentifier=linux-amd64).

We should try to tackle this only in the CLI handling of these options, because we are unable to handle them in the build itself. By the time the CLI invokes MSBuild, the RID (if any) has been set as a MSBuild Global Property, so it's unable to be modified.

[richlander]

I forgot about -a for this. That's actually a great point. We should probably switch our examples to -a since that's simpler. As long as SDK and runtime match (like Debian and Debian or Alpine and Alpine), everything will work great. Note that I realize that mixing Debian and Ubuntu will also work fine.

[richlander]

In fact, if it made the feature simpler, I think we might be happy with support for just -a. Is that a good idea?

@mthalman

[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.