If you build from sources, this is under Apache 2 license. To be outside of subscription for Docker, you have to adapt the script, and build and host everything yourself.
The self-built version does not limit the number of TCP connections, why would you think that.
Note that the subscriptions are a way to support the project and keep Verne maintained and developed in the future. You can also support the VerneMQ project by Github Sponsoring.
Regards,
André