image: add user agent to Docker registry options

[stefanb2]

Set the string to "libpod/VERSION" so that we don't use the unspecific default of "Go-http-client/xxx".

Fixes #3788

This implements option (2) from the suggested list.

[openshift-ci-robot]

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[mheon]

/ok-to-test
/approve

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mheon, stefanb2

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[rhatdan]

/lgtm
Eventually we should allow this to be passed into libpod, since we might want to use podman/VERSION, to differentiate cri-o/VERSION, or toolbox/VERSION.

For now this is much better.