[Feature] Support multi context builds

[Neirda24]

Hi.

Actual behavior Kaniko does not seem to support either multiple build contexts nor named ones.

Expected behavior See this article https://www.docker.com/blog/dockerfiles-now-support-multiple-build-contexts/

that allows syntax such as COPY --from=some_context where some_context is set during docker build

To Reproduce Run kaniko

Additional Information

FROM scratch

COPY . .
COPY --from=my_context . /other-directory

$ docker build --build-context my_context=/some/directory /some/other/directory

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing	- [x]
Please check if the build works in docker but not in kaniko	- [x]
Please check if this error is seen when you use --cache flag	- [ ]
Please check if your dockerfile is a multistage dockerfile	- [ ]

[igor-sadchenko]

Any updates here? @aaron-prindle @JeromeJu