How can I run wordpress docker-image using nginx-proxy?

Question

I am trying to run a WordPress app inside of a docker container on Ubuntu VPS using Nginx-Proxy.

First I run the nginx-proxy server using the following command

docker run -d \
-p 80:80 \
-p 443:443 \
--name proxy_server \
--net nginx-proxy-network \
-v /etc/certificates:/etc/nginx/certs \
-v /var/run/docker.sock:/tmp/docker.sock:ro \
jwilder/nginx-proxy

Then I run the mysql database server using the following command

docker run -d \
--name mysql_db \
--net nginx-proxy-network \
-e MYSQL_DATABASE=db1 -e \
MYSQL_USER=db1 -e \
MYSQL_PASSWORD=db1 -e \
MYSQL_ROOT_PASSWORD=db12 \
-v mysql_server_data:/var/lib/mysql \
mysql:latest

I am able to verify that MySql server is running by connecting to it using the following command

root:~# docker exec -it mysql_db /bin/bash
root@dd7643384f76:/# mysql -h localhost -u root -p
mysql> show databases;

Now that nginx-proxy and mysql_db images are running, I want to proxy the WordPress image on the usa.mydomain.com. To do that, I run the following command

docker run -d \
--name wordpress \
--expose 80 \
--net nginx-proxy-network \
-e DEFAULT_HOST=usa.mydomain.com \
-e WORDPRESS_DB_HOST=mysql_db:3306 \
-e WORDPRESS_DB_NAME=db1 \
-e WORDPRESS_DB_USER=db1 \
-e WORDPRESS_DB_PASSWORD=db1 \
-v wordpress:/var/www/html \
wordpress:latest

I can see all 3 container running by executing docker ps -a

However, when I browser http://usa.mydomain.com I get HTTP error 503

503 Service Temporarily Unavailable nginx/1.17.5

I validated that usa.mydomain.com is pointing to the server's IP address by doing the following using the command line my my machine.

ipconfig /flushdns
ping usa.mydomain.com

Even when I try to browse my server's ip address I get the same 503 error.

What could be causing this issue?