[Request]: DOCS: BUILDING.md should include instructions for using locally build init filesystem.

[jglogan]

Feature or enhancement request details

Right now we're missing the instructions for how to set the system property to use the initfs that make all will create when using a local containerization repo:

container system property set image.init vminit:latest

and how to back that change out when your done locally editing the containerization package:

container system property clear image.init

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