fix: Change alpine version grab to include a colon in the updater

[ciarams87]

Proposed changes

See https://github.com/nginxinc/kubernetes-ingress/runs/7316328564?check_suite_focus=true

The grep pulls the version from the opentracing image instead of the alpine image at the moment (resulting in an image tag of nginx-1.23.0-alpine instead of 1.23.0-alpine) causing failures.

The fix is to put a colon in so that get the right image tag.

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
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