adding skaffold version to the docker push user agent

[balopat]

Fixes #960.

[codecov-io]

Codecov Report

Merging #1260 into master will increase coverage by 1.87%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1260      +/-   ##
=========================================
+ Coverage   42.42%   44.3%   +1.87%     
=========================================
  Files          96     104       +8     
  Lines        4247    4623     +376     
=========================================
+ Hits         1802    2048     +246     
- Misses       2269    2366      +97     
- Partials      176     209      +33

Impacted Files	Coverage Δ
pkg/skaffold/docker/client.go	62.1% <100%> (+3.96%)	⬆️
pkg/skaffold/schema/versions.go	47.22% <0%> (-17.65%)	⬇️
cmd/skaffold/app/cmd/runner.go	0% <0%> (ø)	⬆️
pkg/skaffold/schema/v1alpha2/upgrade.go	68.88% <0%> (ø)
pkg/skaffold/schema/v1alpha4/upgrade.go	58.62% <0%> (ø)
pkg/skaffold/schema/v1alpha1/upgrade.go	70.31% <0%> (ø)
pkg/skaffold/schema/v1alpha1/config.go	57.14% <0%> (ø)
pkg/skaffold/schema/v1alpha4/defaults.go	47.05% <0%> (ø)
pkg/skaffold/schema/v1alpha2/defaults.go	73.62% <0%> (ø)
pkg/skaffold/schema/v1alpha2/config.go	45.45% <0%> (ø)
... and 2 more

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