Shiny app does not appear when deployed using Shinyproxy - r

Question

I'm trying to learn how to deploy a shiny app using Shinyproxy, and I'm using the templated "euler app" (from this repo), but the application does not appear when I navigate to http://localhost:4445. Here's the most similar question I could find, but unfortunately not helpful to my issue: link.
Background
All installations seem fine, and I successfully installed Docker and Java.
The Dockerfile and Docker image work locally, no issues there. The command docker run --rm -p 3838:3838 shiny-euler-app works.
Here is my Dockerfile (copied from the repo):
FROM openanalytics/r-base
MAINTAINER Tobias Verbeke "tobias.verbeke#openanalytics.eu"
# system libraries of general use
RUN apt-get update && apt-get install -y \
sudo \
pandoc \
pandoc-citeproc \
libcurl4-gnutls-dev \
libcairo2-dev \
libxt-dev \
libssl-dev \
libssh2-1-dev \
libssl1.1
# system library dependency for the euler app
RUN apt-get update && apt-get install -y \
libmpfr-dev
# basic shiny functionality
RUN R -e "install.packages(c('shiny', 'rmarkdown'), repos='https://cloud.r-project.org/')"
# install dependencies of the euler app
RUN R -e "install.packages('Rmpfr', repos='https://cloud.r-project.org/')"
# copy the app to the image
RUN mkdir /root/euler
COPY euler /root/euler
COPY Rprofile.site /usr/lib/R/etc/
EXPOSE 3838
CMD ["R", "-e", "shiny::runApp('/root/euler')"]
As well, Shinyproxy works fine with the default openanalytics/shinyproxy-demo Docker image, as you can see:
Problem
The issue I have is when I try and supply a different Shiny app and its accompanying application.yml. Here is the application.yml file I'm using (I've tried to make it as basic as possible, with no authentication, etc):
proxy:
title: Standalone Docker Engine
port: 4445
authentication: none
docker:
url: http://localhost:2375
specs:
- id: euler
display-name: Euler's number
container-cmd: ["R", "-e", "shiny::runApp('/root/euler')"]
container-image: shiny-euler-app
Unfortunately, when I run java -jar shinyproxy-2.4.2.jar (in the directory which contains the shinyproxy-2.4.2.jar file and the application.yml file) I get this blank webpage:
For some reason, I am able to access the Shinyproxy webpage, but the Dockerized Shiny app does not appear.
Would really appreciate any helpful suggestions on where/how I could try and solve this issue. Thanks!