Update command syntax for setting INFRA_IF_NAME

[tim-vk]

Include INFRA_IF_NAME with the bash command, it is now copy-past-able.

[gemini-code-assist]

Code Review

This pull request updates the border router Docker build guide to correctly pass the INFRA_IF_NAME environment variable to the bash command instead of the curl command. No review comments were provided, and the change looks correct.

[jwhui]

Thanks! 👍🏼