Support '--squash' option in 'docker build'

[savyasachi9]

Background

docker 'build' supports '--squash' as an option which helps in reducing the size of docker image by squashing multiple layers into one
--squash Squash newly built layers into a single new layer

Feature request

It'd be great if Skaffold support this too, so I played with Skaffold code by referencing below PRs and got '--squash' working

• Support '--ssh' option in 'docker build'
• Support docker build --secret flag

I am submitting PRs to add support for '--squash' since I already have working code for it

• i still have to add error for 'google cloud build/gcb' which doesn't support '--squash' I believe

here's the PR :
#5078