How to specify the external port for an automatically started Docker container?

[Monte-Christo]

The MongoDb driver aspire.mongodb.driver autostarts a Mongo container with a random external port. I would like to specify a fixed port mapping to make development simpler, but

var mongodb = builder.AddMongoDB("mongodb").WithServiceBinding(hostPort: 50217);
builder.AddProject<AspireSample_WorkerService>("workerservice").WithReference(mongodb);

throws a System.InvalidOperationException: Sequence contains more than one element at builder.Build().Run();.

Also, how can I suppress the creation of the container, for instance when I already have a Mongo cloud instance to connect to? I have succeeded in connecting to that instance, but the now superfluous container is still started every time the app runs.