Docker container not running

I’m following this Dockerized Flask App guide with success!

https://www.digitalocean.com/community/tutorials/how-to-build-and-deploy-a-flask-application-using-docker-on-ubuntu-18-04

I am now adapting my own code, and started with two small changes of setting the base image to FROM python:3.7.3-stretch and adding opencv-python==4.4.0.42 as a requirement (I did this as a docker image on my own local machine successfully). On the Droplet, I see that the container is being successfully created but won’t continue to run even though it’s being invoked with a -d argument. Here’s the output:

#docker images
REPOSITORY                   TAG                   IMAGE ID            CREATED             SIZE
nlp.test                     latest                226105eb5bb1        20 minutes ago      1.23GB

root@NLP-FlaskApp-docker-ubuntu-s-1vcpu-2gb-nyc1-01:/var/www/nlpApp# docker run -d -p 56734:80 --name=nlp.test -v $PWD:/app nlp.test
bcb213b1c1ce5e40f29092f0afbcf6b5e1fa1b76e7922a548af806cd9e43dfec
root@NLP-FlaskApp-docker-ubuntu-s-1vcpu-2gb-nyc1-01:/var/www/nlpApp# docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

So nothing shows up as running after the command. I also tried running the same command with different port numbers that I checked are open as well as using the start.sh file given in the guide above.

Please let me know of any debug ideas! Thanks