push: Add insecure-registry flag

[guilhermebr]

related to #85

moby/buildkit already accept push to insecure registries.

this PR just add the flag to use it.

[codecov-io]

Codecov Report

Merging #99 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #99      +/-   ##
=========================================
- Coverage    0.16%   0.16%   -0.01%     
=========================================
  Files          11      11              
  Lines         618     619       +1     
=========================================
  Hits            1       1              
- Misses        616     617       +1     
  Partials        1       1

Impacted Files	Coverage Δ
push.go	0% <0%> (ø)	⬆️

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[jessfraz]

can you update the readme as well for that command :)