ShinyProxy app can't see other port

I’ve got my Shinyproxy app working, but I’m running into an issue.

In my Shiny app, I want to use RSelenium to scrape data from a dynamically loaded webpage. Previously, before using Shinyproxy and I was just using shiny server this worked. In my R file I have this code:

library(RSelenium)
remDr <- remoteDriver(port = 4445L)
remDr$open()

I use docker to allow this to happen by pulling the docker image I need and doing the following:

sudo docker run -d -p 4445:4444 selenium/standalone-firefox:2.53.0

Then when I run my app, it works.

However, now that I am trying to use ShinyProxy, when I test the app using

sudo docker run -p 3838:3838 openanalytics/shinyproxy-template R -e ‘shiny::runApp("/root/Shiny")’

… the R script runs and throws the error:

Error in checkError: Undefined error in httr call. httr output: Failed to connect to localhost port 4445: Connection refused.

I’m assuming this is because the app can’t see this port now that it is container form?

Thanks in advance!
Evan