fix: dockerimage correctly set auth if arch unspecified

[olblak]

Fix #1613

The authentication options weren't defined if the plugin was used without specifying the architecture in the setting which is solved with the current pullrequest.

I tested this PR using

sources:
  default:
    kind: dockerimage
    spec:
      image: ghcr.io/olblak/charts/upgrade-responder

conditions:
  default:
    kind: dockerimage
    spec:
      image: ghcr.io/olblak/charts/upgrade-responder
      tag: v0.1.5

which is a private docker image

Test

To test this pull request, you can run the following commands:

cd pkg/plugins/resources/dockerimage/
go test

Additional Information

Tradeoff

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Potential improvement

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