Fix: try empty credentials as last option

[matejvasek]

Changes

• 🐛 Fix using empty credentials for docker.io, the issue is that docker.io recognizes empty credentials (as opposed to for instance quay.io), but only for read operations. So authentication passes but not authorization.

/kind bug

bugfix: using empty credentials for docker.io

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matejvasek

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[codecov]

Codecov Report

Merging #1100 (64a62ec) into main (afd224f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1100   +/-   ##
=======================================
  Coverage   44.94%   44.94%           
=======================================
  Files          64       64           
  Lines        8502     8502           
=======================================
  Hits         3821     3821           
  Misses       4298     4298           
  Partials      383      383           

Impacted Files	Coverage Δ
docker/creds/credentials.go	69.25% <100.00%> (ø)

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[lance]

/lgtm
/hold for tests

[lance]

/unhold