docs: document from platform

[tonistiigi]

FROM --platform was added in moby/buildkit#464 but there was no documentation.

@thaJeztah @tiborvass

Signed-off-by: Tonis Tiigi tonistiigi@gmail.com

[tiborvass]

Suggested change

	Optional flag `--platform` can be used to specify the platform of the image in
	The optional flag `--platform` can be used to specify the platform of the image in

[codecov-io]

Codecov Report

Merging #2003 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2003      +/-   ##
==========================================
- Coverage   56.79%   56.78%   -0.01%     
==========================================
  Files         311      311              
  Lines       21836    21832       -4     
==========================================
- Hits        12402    12398       -4     
  Misses       8519     8519              
  Partials      915      915

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "platform-docs" git@github.com:tonistiigi/docker-cli.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354512184
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[AkihiroSuda]

Necessity for enabling buildkit-mode needs to be mentioned?

[tonistiigi]

@AkihiroSuda There is code to handle this without buildkit as well. I wouldn't recommend it though as all of the useful patterns(cross-compile) are only performant on buildkit.