@Mirabito, Massimo (Max) Thanks for posting your question in Microsoft Q&A, apologize for any inconvenience caused on this.
Based on the shared information, I understand that you are trying to SSH into the app service that is running on custom container. Correct my if my understanding is wrong.
Please follow the below steps to Configure & SSH into Linux docker images app service.
- Enable SSH in custom docker images in App service.
- SSH into the Linux docker images. Also, note that Open SSH is currently in preview.