Installing connectors in self-managed desktop modeler

I’m completely new to all of this.
I’m trying to run a self-managed version using the docker compose.
To get the out-of-the-box connectors to work I simply had to add the element-templates to the folder of the camunda desktop modeler. But if I want to add any other connector I can’t get it to work. e.g. this one: camunda-connectors/connector-smtp at main · Infosys/camunda-connectors · GitHub
I build it using: mvn clean package, I then put the .jar in the docker container. (I add a volume in the docker_compose.yml), but then when I start the container I get this error in the connectors container:

Error: Could not find or load main class
Caused by: java.lang.ClassNotFoundException:

Any ideas what im doing wrong?