fix: add variant to manifest entry

[thesayyn]

part of #362

[alexeagle]

Could you help me understand why it's an improvement? It feels approx the same performance and complexity overall

[thesayyn]

Could you help me understand why it's an improvement? It feels approx the same performance and complexity overall

the json.encode_ident does not provide any performance improvement over the templated json, it is purely nice to have to allow more properties to be added to the platform sub-object conditionally without templating tricks. https://github.com/opencontainers/image-spec/blob/main/image-index.md#image-index-property-descriptions
I can revert that.

That said the use of write_file is unnecessary here and adds some performance overhead. eg: spawning some actions to write a file which is pure overhead given that we already write the same content to the BUILD file.

[thesayyn]

actually, now that I think about it more, I will keep that part the same just make it more ergonomic. eg: instead of two templates, just use one.

[thesayyn]

@alexeagle i took back the templated strings back and made an util function that can add variant as well. now we match crane pull 100% of the cases.

now we have the ability to write a starlark unit test for the util function. see #138