Fix onbuild analysis

[dgageot]

Current code fails to retrieve remote image for python:3-slim

[codecov-io]

Codecov Report

Merging #1491 into master will increase coverage by 0.16%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1491      +/-   ##
==========================================
+ Coverage   44.87%   45.04%   +0.16%     
==========================================
  Files         115      115              
  Lines        4771     4771              
==========================================
+ Hits         2141     2149       +8     
+ Misses       2409     2397      -12     
- Partials      221      225       +4

Impacted Files	Coverage Δ
pkg/skaffold/docker/parse.go	74.88% <100%> (ø)	⬆️
pkg/skaffold/docker/remote.go	24.24% <50%> (+24.24%)	⬆️

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[nkubala]

Well, there it is....I said I would take responsibility if this broke something, so let it be known that this one is all my fault 🤦‍♂️

@dgageot thanks for fixing, though I'm still a bit unsure this is the right way to do this. We can address in another PR, but I think we might want to try using latest if we don't have an explicit tag. Otherwise this can return strange results w.r.t. the image registry (e.g. sometimes it will give you back index.docker.io in places you don't want it). I'll need to play around with it some more though, so for now this LGTM