Also possible to use Enterprise license

import os
from testcontainers.iris import IRISContainer

license_key = os.path.abspath(os.path.expanduser("~/iris.key"))
image = 'containers.intersystems.com/intersystems/iris:2023.3'
container = IRISContainer(image, username="demo", password="demo", namespace="demo", license_key=license_key)
container.with_exposed_ports(1972, 52773)
container.start()
print('SQLAlchemy URL', container.get_connection_url())
print('Username', container.username)
print('Password', container.password)

You already found port 52773

IRIS does not have http://localhost:52773/RPCNAME

Where did you get it RPCNAME? How did you configure it in IRIS? What else would you expect it to answer?

I tried to find anything about VistA RPC Broker, and found only an installation guide from 1997, It's a year when Caché was released. 

So, you are asking about VistA RPC Broker on IRIS, I don't know how it's possible 

And why it's an issue with IRIS, not with VistA RPC Broker