Add a new --platform or --target flag to pack build command to avoid breaking M1 workflow users after multi-platform is released

[jjbustamante]

Description

Right now, multi-arch builders aren't too prevalent in the CNB ecosystem. Most builders are linux/amd64. But with the release of pack 0.34 (enables multi-arch builder creation) this might change.

Today, it is possible to specify the os/arch for a build by specifying the (not multi-arch) builder that supports the desired platform. When more builders are multi-arch, this selection mechanism goes away.

Specifically, M1 users with emulation enabled might lost the ability to build linux/amd64 images because when using a multi-arch builder, the linux/arm64 version will be automatically selected.

Proposed solution

We should support pack build --platform <my desired platform>

Describe alternatives you've considered

pack build --target <my desired platform> - but this probably makes sense to add when CNB supports cross-platform builds. As an example, pack build --platform <my desired build platform> --target <my desired runtime platform> could become a thing.

Additional context

• This feature should be documented somewhere

[jjbustamante]

cc @natalieparellano

We need to fill the description.