Getting Ember frontend and Flask backend to communicate through separate Docker containers

I have dockerized a backend webapp built with Flask. The CMD line of the Dockerfile runs python app.py which internally calls app.run(host='0.0.0.0'), so that the app is running (within the docker container) at 0.0.0.0:5000. I then run the built container with

$ docker run -p 5000:5000 -it my-image:latest

The frontend is built with ember. Within the .ember-cli file, proxy is set to http://localhost:5000. When I run $ ember serve locally, the app runs smoothly at localhost:4200, communicating with the backend running from the docker image. However, I also want to run the frontend on a docker image (based on an apache image). My Dockerfile:

FROM httpd:2.4-alpine
COPY ./dist/ /usr/local/apache2/htdocs/
COPY ./.htaccess /usr/local/apache2/htdocs/

The .htaccess file is copied directly from Ember - Deploying (as per the Servers section):

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^index\.html$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule (.*) index.html [L]
</IfModule>

I then do the following:

$ ember b --prod  # build the app for production (stored in ./dist/)
$ docker build -t my-frontend:latest .
$ docker run -dit --name my-frontend -p 8080:80 my-frontend:latest

I then go to localhost:8080, and can see my app's loading wheel, which means this image is working in isolation. However, I continue to get 404 errors when it tries to access the backend.

What am I doing wrong that the containers can't communicate with each other?