Add example to use docker buildx via the custom builder

[briandealwis]

Description
This example demonstrates how to use docker buildx as a Skaffold custom builder.

The one downside is that it requires setting up a docker buildx builder instance as a separate step. Something like:

docker buildx create --name skaffold-builder --platform linux/arm64,linux/amd64

We could create a more sophisticated build script that would examine the existing builder instances, but that's a bit more involved.

[codecov]

Codecov Report

Merging #5426 (ef70183) into master (c1b3f76) will increase coverage by 0.04%.
The diff coverage is n/a.

❗ Current head ef70183 differs from pull request most recent head a08cdb5. Consider uploading reports for the commit a08cdb5 to get more accurate results

@@            Coverage Diff             @@
##           master    #5426      +/-   ##
==========================================
+ Coverage   71.34%   71.39%   +0.04%     
==========================================
  Files         399      397       -2     
  Lines       14508    14540      +32     
==========================================
+ Hits        10351    10381      +30     
- Misses       3385     3388       +3     
+ Partials      772      771       -1     

Impacted Files	Coverage Δ
pkg/skaffold/test/structure/structure.go	86.79% <0.00%> (-13.21%)	⬇️
pkg/skaffold/runner/new.go	67.58% <0.00%> (-1.70%)	⬇️
pkg/skaffold/test/structure/error.go	100.00% <0.00%> (ø)
pkg/skaffold/test/structure/types.go
pkg/skaffold/test/error.go
pkg/skaffold/test/test.go
pkg/skaffold/test/test_factory.go	78.94% <0.00%> (ø)
pkg/skaffold/util/tar.go	56.00% <0.00%> (+5.33%)	⬆️
cmd/skaffold/app/cmd/parse_config.go	78.33% <0.00%> (+6.90%)	⬆️

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[tejal29]

do we also need to add the docker buildx use skaffold-builder command.
or add the --use flag to docker buildx create --name skaffold-builder --platform linux/amd64,linux/arm64 command.

[briandealwis]

I have a slight preference to explicitly specifying the builder with --builder as the message if the user hasn't created a builder and continues to use the default docker builder is cryptic:

error: multiple platforms feature is currently not supported for docker driver. Please switch to a different driver (eg. "docker buildx create --use")

Whereas explicitly specifying --builder xxx where xxx doesn't exist fails with a slightly more coherent message:

error: open /Users/bdealwis/.docker/buildx/instances/foo: no such file or directory

[briandealwis]

PTAL @PriyaModali

changes addressed