bake: ensure --builder is wired from root options

[errordeveloper]

Fix #320

[tonistiigi]

cc @cpuguy83

[cpuguy83]

Not sure how I missed this.

LGTM

[cpuguy83]

Before change:

$ docker buildx --builder=foo bake
[+] Building 0.5s (5/9)
[+] Building 0.7s (9/13)

After change:

 $ ./buildx --builder=foo bake
Error: open /Users/cpuguy83/.docker/buildx/instances/foo: no such file or directory
Usage:
  ./buildx bake [OPTIONS] [TARGET...] [flags]

Aliases:
  bake, f

Flags:
  -f, --file stringArray   Build definition file
  -h, --help               help for bake
      --load               Shorthand for --set=*.output=type=docker
      --no-cache           Do not use cache when building the image
      --print              Print the options without building
      --progress string    Set type of progress output (auto, plain, tty). Use plain to show container output (default "auto")
      --pull               Always attempt to pull a newer version of the image
      --push               Shorthand for --set=*.output=type=registry
      --set stringArray    Override target value (eg: targetpattern.key=value)

Global Flags:
      --builder string   Override the configured builder instance

[errordeveloper]

On Wed, 8 Jul 2020, 9:13 pm Brian Goff, ***@***.***> wrote: ***@***.**** approved this pull request. Not sure how I missed this.

I think this is just because root options only holds this flag, and common options get you think you got them all....