I had missed that you are using a remote MySQL server.
So in that case, the docker-compose.yml and cms_custom-ports.yml files play no part and can even be removed to save confusion.
The only file we’re interested in is config.env (based on the remote mysql template), and cms_remote-mysql.yml.
Your MySQL hostname is the problem. MySQL cannot be running on “localhost” as the container sees it. The container runs on an isolated network separate to your host machine. You’ll need to specify the IP address of your MySQL server, and ensure that the MySQL server is listening on the network.
So for example, if your MySQL server is listening on 192.168.0.10, port 3307, then those are the values you would need to put in to the config.env file. By default, MySQL doesn’t bind to any network interfaces, so you’d need to amend your MySQL server configuration to allow that, and then protect connections to it with appropriate firewall rules.
You are correct that before the import can succeed, you also need to create an empty database with the name you’ve chosen (digsign) and ensure that the username and password you’ve given are permitted to access that database, from a remote location. So your grant will need to be for 'routers'@'%' rather than 'routers'@'localhost'.
The digsign user should not be a full database admin. It only needs access to your Xibo database and so that’s what it should be granted access to.