Add ARM platform support for local CLI builds

[lucataco]

Summary

• thread a configurable build/run platform through the CLI and Docker build pipeline
• default local image pulls, builds, and runs to the host architecture instead of forcing amd64
• adjust Dockerfile generation for arm64, including library paths, venv setup, and cog base image fallback behavior

[michaeldwan]

Nice this is a common request. I'll help get this working once we finish the major changes to the in-container runtime. The important part is the CLI only is arm64, we do need the container to be amd64 until all the dependencies work correctly on arm64, like torch and cuda. tbh that's not super likely to be easy, but the exercise will help make the container vs cli vs sdk and dep logic arch aware, which is a great thing.