There's a service that I want to use, however for reasons, it no longer has any builds available. Consequently, I am thinking of building it myself. How does one go about doing that and then afterwards, how do I get it up on Docker hub? Can I just create an account and upload?
yes. it's going to completely depend on the project, but you can build it, then push it to dockerhub, though I wouldn't push it to a public repo unless the license permits it. else you can just build it locally and push it to your own private dockerhub or to a container store like github or ECR.