fix(ConfigFile): Add Variant

[lippertmarkus]

Signed-off-by: Markus Lippert lippertmarkus@gmx.de

By mutating an image, the configfile is copied. When the source image is e.g. linux/arm/v7 the variant is currently not copied as its not part of the ConfigFile struct.

This field is usually part of the config https://github.com/opencontainers/image-spec/blob/main/image-index.md#platform-variants:
crane config mcr.microsoft.com/dotnet/runtime-deps:6.0-alpine-arm64v8

{
     ...
     "architecture": "arm64",
     "os": "linux",
+    "variant": "v8"
}

[imjasonh]

Looks like this is indeed specified: https://github.com/opencontainers/image-spec/blob/main/config.md

• variant string, OPTIONAL
  The variant of the specified CPU architecture. Configurations SHOULD use, and implementations SHOULD understand, variant values listed in the Platform Variants table.