Instead of documenting the error in a warning block, I'd suggest we keep a primary callout block with directly the good instructions you wrote.

We could add a very brief explanation why localhost:5000 doesn't work.

Perhaps this?

<div class="callout primary">
    <h3>Pull a container</h3>
    <p>
        Pull with <code>registry.nasqueron.org/image_name</code>.
    </p>
    <p>
        Always use <code>registry.nasqueron.org</code> as Docker pulls by default in HTTPS.
        If you try to pull from `localhost:5000`, you won't get a certificate.
    </p>
</div>

Follow-up change

In another commit / differential, it could be a good idea to increase the column width, so we avoid the débordemnent of the monospaced text in both Add a container and Pull a container section.