Don't push all tags when sha256 builds just one

[michaelfig]

I don't yet know how to describe the problem, except for that sha256 sends thousands of pushes if you use it with an image that doesn't have its own tag.

Forcing the tag to :skaffold (:latest would work, too) makes it behave properly.

[codecov-io]

Codecov Report

Merging #1634 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1634      +/-   ##
==========================================
+ Coverage    46.9%   46.95%   +0.05%     
==========================================
  Files         119      119              
  Lines        5134     5139       +5     
==========================================
+ Hits         2408     2413       +5     
  Misses       2478     2478              
  Partials      248      248

Impacted Files	Coverage Δ
pkg/skaffold/build/tag/sha256.go	70% <100%> (+30%)	⬆️

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[dgageot]

@michaelfig Could you create a ticket with a procedure to reproduce? I couldn't reproduce the issue here.

[michaelfig]

@dgageot working on finding a simple example for a new issue.

[michaelfig]

@dgageot #1638 describes this problem in detail, with minimal steps to reproduce.

On my system, I have a lot of tags on one image, so it's especially slow and onerous to push them all.