Will I need a SQL license for this? No-one will ever use this copy for any work.
As per SQL Server Licensing: High Availability and Disaster Recovery benefits if you have a license and have also purchases Software Assurance (SA)
you are allowed to run a PASSIVE instance. A passive instance is one on which you do not run query. In your case since you would only be restoring I believe you can go ahead (of course if you have purchased SA).
I always urge OP to get expert opinion on licensing from MS licensing expert.