I can create an FTP-server using docker according to this. I wonder whether it works in the azure web app for containers. If the answer is yes, how could make it works?
By the way, I've tried it, according to the steps from the link, I have to create users. but I don't know how to connect the container's linuxOS.
Generally, the FTP server should need to open multiple ports as the document you provided shows. But Azure Web App service only can open 80 and 443 port. And you would not be a whole controller for it. So, if you want to deploy an FTP server, the Azure Web App service is not a good choice. Even if it can run the FTP image. And the VM is recommended.
By the way, if you want to connect to the container's LinuxOS of Web App, you need to enable the SSH feature in the image before you deploy it into the Web App. You can follow the steps about How to enable the SSH in the Web App.