Added dockerBuildInit option

[jroper]

See #1257.

[guizmaii]

Hi,

Tried to activate this new option and rebuilt my image but I get the following error:

[error] unknown flag: --init
[error] See 'docker buildx build --help'.
[info] Removing intermediate image(s) (labeled "snp-multi-stage-id=035fd77f-20c7-432f-ae31-93913656b499") 
[info] Total reclaimed space: 0B
[error] stack trace is suppressed; run last app-embedded-docker / Docker / publishLocal for the full output
[error] (app-embedded-docker / Docker / publishLocal) Nonzero exit value: 125
[error] Total time: 51 s, completed 31 Aug. 2021, 9:57:13 pm

I'm on Mac, is it only available on Linux? 🤔

I have an up-to-date installation of Docker: