I'm trying to use docker-compose and kubernetes as two different solutions to setup a Django API served by Gunicorn (as the web server) and Nginx (as the reverse proxy). Here are the key files:

default.tmpl (nginx) - this is converted to default.conf when the environment variable is filled in:

upstream api {
    server ${UPSTREAM_SERVER};
}

server {
    listen 80;

    location / {
        proxy_pass http://api;
    }

    location /staticfiles {
        alias /app/static/;
    }
}

docker-compose.yaml:

version: '3'
services:
  api-gunicorn:
    build: ./api
    command: gunicorn --bind=0.0.0.0:8000 api.wsgi:application
    volumes:
      - ./api:/app

  api-proxy:
    build: ./api-proxy
    command: /bin/bash -c "envsubst < /etc/nginx/conf.d/default.tmpl > /etc/nginx/conf.d/default.conf && exec nginx -g 'daemon off;'"
    environment:
      - UPSTREAM_SERVER=api-gunicorn:8000
    ports:
      - 80:80
    volumes:
      - ./api/static:/app/static
    depends_on:
      - api-gunicorn

api-deployment.yaml (kubernetes):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: release-name-myapp-api-proxy
spec:
  replicas: 1
  selector:
    matchLabels:
      app.kubernetes.io/name: myapp-api-proxy
  template:
    metadata:
      labels:
        app.kubernetes.io/name: myapp-api-proxy
    spec:
      containers:
        - name: myapp-api-gunicorn
          image: "helm-django_api-gunicorn:latest"
          imagePullPolicy: Never
          command:
            - "/bin/bash"
          args:
            - "-c"
            - "gunicorn --bind=0.0.0.0:8000 api.wsgi:application"
        - name: myapp-api-proxy
          image: "helm-django_api-proxy:latest"
          imagePullPolicy: Never
          command:
            - "/bin/bash"
          args:
            - "-c"
            - "envsubst < /etc/nginx/conf.d/default.tmpl > /etc/nginx/conf.d/default.conf && exec nginx -g 'daemon off;'"
          env:
            - name: UPSTREAM_SERVER
              value: 127.0.0.1:8000
          volumeMounts:
            - mountPath: /app/static
              name: api-static-assets-on-host-mount
      volumes:
        - name: api-static-assets-on-host-mount
          hostPath:
            path: /Users/jonathan.metz/repos/personal/code-demos/kubernetes-demo/helm-django/api/static

My question involves the UPSTREAM_SERVER environment variable.

For docker-compose.yaml, the following values have worked for me:

• Setting it to the name of the gunicorn service and the port it's running on (in this case api-gunicorn:8000). This is the best way to do it (and how I've done it in the docker-compose file above) because I don't need to expose the 8000 port to the host machine.
• Setting it to MY_IP_ADDRESS:8000 as described in this SO post. This method requires me to expose the 8000 port, which is not ideal.

For api-deployment.yaml, only the following value has worked for me:

• Setting it to localhost:8000. Inside of a pod, all containers can communicate using localhost.

Are there any other values for UPSTREAM_SERVER that work here, especially in the kubernetes file? I feel like I should be able to point to the container's name and that should work.