Fix iframe in docker container

[jnumainville]

Fixes #10
Verified by creating a docker container:

docker run -p 10000:8888 -p 8080:1234 ghcr.io/deephaven/deephaven-kube-demo-worker-jupyter:main

Then running the following code in a jupyter notebook in the docker container:

from deephaven_server import Server

s = Server(port=1234, jvm_args=["-Xmx10g"])
s.start()

from deephaven_ipywidgets import DeephavenWidget
from deephaven.plot.figure import Figure
from deephaven import empty_table
from deephaven import time_table
from deephaven import ugp

static_table = empty_table(100).update(["X = i", "Y = Math.sin(0.1 * X)"])

import os
os.environ['DEEPHAVEN_IPY_URL'] = "http://localhost:8080/"

static_widget = DeephavenWidget(static_table)
display(static_widget)