Implement crane digest using a HEAD request

[imjasonh]

Before

$ crane digest ubuntu
sha256:bc2f7250f69267c9c6b66d7b6a81a54d3878bb85f1ebb5f951c896d13e6ba537

After

$ go  run  ./cmd/crane/ digest ubuntu
sha256:bc2f7250f69267c9c6b66d7b6a81a54d3878bb85f1ebb5f951c896d13e6ba537

This wouldn't count as a "pull" from Dockerhub, which might be helpful in some cases.

[codecov-io]

Codecov Report

Merging #784 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #784      +/-   ##
==========================================
+ Coverage   75.91%   75.94%   +0.02%     
==========================================
  Files         102      102              
  Lines        4139     4144       +5     
==========================================
+ Hits         3142     3147       +5     
  Misses        546      546              
  Partials      451      451              

Impacted Files	Coverage Δ
pkg/crane/digest.go	100.00% <100.00%> (ø)
pkg/crane/get.go	100.00% <100.00%> (ø)

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