Remove `docker pull` prefix when copying a tag from the registry

What does this MR do?

Resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/46660

Change the copy-to-clipboard mechanism of the registry to stop prepending docker pull to every copy.

I think that users are more likely to use the tag for something else rather then pulling it through the Docker CLI. Other possible uses:

• Calling docker run
• Pasting it in a template, message, docs, script

Not too sure about the syntax, can the wrapping "" be omitted now that the prefix is gone?

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created There was none before
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers