Add check for usage of --platform= flag in Dockerfile FROM (FromPlatformFlagConstDisallowed)

[thompson-shaun]

Specifying --platform= with a constant value in the Dockerfile FROM instruction forces the Image to build only one target platform. This has some drawbacks:

• Make it no longer possible to build a multi-platform Image from this Dockerfile.
• The platform that you build on must be the same as the platform specified in --platform=

The recommended approach include:

• Omit FROM --platform in the docker file and to specify in buildx --platform= during the build if control of the image platform target is needed.
• Use the $BUILDPLATFORM or other combination of variables for --platform in a FROM
  stage name should include a valid OS or ARCH
• --platform flag should not be used with a constant value, unless stage name also contains os or arch

Reference

• BUILDPLATFORM — matches the current machine. (e.g. linux/amd64)
• BUILDOS — os component of BUILDPLATFORM, e.g. linux
• BUILDARCH — e.g. amd64, arm64, riscv64

[SampsonCrowley]

how can we silence this warning?

Make it no longer possible to build a multi-platform Image from this Dockerfile.

Not everyone wants a multi-platform image. some things only ever are ran from one platform no matter where it's built from

The platform that you build on must be the same as the platform specified in --platform=

I don't think that's true. I've built AMD64-only images from arm OSX without an issue