docs: add docker-env prompt tip

[iacker]

Summary

• add a new tip to the image pushing handbook showing how to surface MINIKUBE_ACTIVE_DOCKERD in the shell prompt
• document a simple bash / zsh example for making the active minikube Docker target visible
• note that users with custom prompts should adapt the snippet rather than replacing their prompt configuration verbatim

Validation

• cd site && npm ci
• independent content review against issue discussion and existing docs
• site build target was not available via make -C site build in this checkout

Fixes #6489

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @iacker. Thanks for your PR.

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[minikube-bot]

Can one of the admins verify this patch?