Assign the gitlab--duo runner tag in the Bedrock BYOM guide

What does this MR do?

Step 7 registers the runner with --tag-list "docker". With a runner authentication token, gitlab-runner register exits with a fatal error when given --tag-list, because tags are set when the runner is created (Changes to the gitlab-runner register command syntax).

The tag is also the wrong one: flow jobs are tagged gitlab--duo (WORKLOAD_TAG) whatever the model provider. On GitLab 19.4.1, a flow job stayed pending with only a docker runner and started within seconds of a gitlab--duo runner registering. @rcain's duo-selfhosted-iac registers its flow runner the same way: the gitlab--duo tag set when the runner is created, and no --tag-list.

The button is now Create instance runner (admin_runners_app.vue:273), and no New instance runner string is left in the UI.

Prepared with help from Claude Code and checked against source and the documentation style guide. Vale at suggestion level reports no new alerts on the changed lines.

Related issues

None.

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