Add dockerfile to custom dependencies

[priyawadhwa]

Allows users to say something like:

- image: my image
  custom:
     buildCommand: ./build.sh
     dependencies:
        dockerfile:
           path: path/to/Dockerfile
           buildArgs:
             - file:foo
           target: secondStage

[codecov-io]

Codecov Report

Merging #2049 into master will increase coverage by 0.24%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #2049      +/-   ##
==========================================
+ Coverage   55.93%   56.17%   +0.24%     
==========================================
  Files         178      180       +2     
  Lines        7689     7768      +79     
==========================================
+ Hits         4301     4364      +63     
- Misses       2973     2987      +14     
- Partials      415      417       +2

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/build/local/local.go	37.31% <0%> (ø)	⬆️
pkg/skaffold/schema/validation/validation.go	96.29% <80%> (-3.71%)	⬇️
pkg/skaffold/build/custom/dependencies.go	85.71% <85.71%> (+7.93%)	⬆️
cmd/skaffold/app/cmd/deploy.go	45.83% <0%> (-29.17%)	⬇️
pkg/skaffold/runner/runner.go	64.77% <0%> (-2.1%)	⬇️
cmd/skaffold/app/cmd/build.go	69.04% <0%> (ø)	⬆️
pkg/skaffold/build/util.go	100% <0%> (ø)	⬆️
pkg/skaffold/config/options.go	90.47% <0%> (ø)	⬆️
pkg/skaffold/build/prebuilt.go
... and 6 more

---

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[tejal29]

LGTM,
I am not aware if current config version is frozen. Please verify before you merge!.

[priyawadhwa]

Yup, we've frozen v1beta9 from the last release and this is making changes to v1beta10.