How to get hostname from one container to another using docker compose?

I have two docker containers. One backend, and the other db (postgres). They both are linked. How do I utilise the backend environment HOST variable in the golang docker container?

From my understanding, both containers have their own IP addresses. I cannot use 'localhost' in the golang container because postgres isn't on localhost, but in an isolated container.

version: "3.7"
services:
backend:
  image: golang:1.16
  build: ./
  working_dir: /app
  volumes:
    - ./backend/:/app
  environment: 
    HOST: db
  command: go run main.go
  ports:
    - 8080:8080
  depends_on: 
    - db
db:
  image: postgres
  restart: always
  environment: 
    POSTGRES_USER: gorm
    POSTGRES_PASSWORD: gorm
    POSTGRES_DB: gorm
  ports:
    - 9920:9920

I've tried researching how to access this variable as well as check Docker tutorials/documentation, but haven't found a solution.

Docker compose does DNS resolution. You should be able to access your database by name.

Remove:

environment: 
  HOST: db

Correct the postgres port to 5432:

db:
  ...
  ports:
    - 5432:5432

You must be able to connect like so:

db := pg.Connect(&pg.Options{
	Addr:     "db:5432",
	User:     "gorm",
	Database: "gorm",
	Password: "gorm",
})

As for environment variables, you can declare and access them like this:

backend:
  environment:
    POSTGRES_USER: gorm
    ...

os.Getenv("POSTGRES_USER")

Docker compose will create a network for your containers where they can communicate and reach each other.

You can make a simple change to your docker compose file by adding a name to your services which will make sure they get the same name each time.

version: "3.7"
services:
  backend:
    image: golang:1.16
    build: ./
    container_name: backend
    working_dir: /app
    volumes:
      - ./backend/:/app
    command: go run main.go
    ports:
      - "8080:8080"
    depends_on: 
      - db
  db:
    image: postgres
    restart: always
    container_name: db
    environment: 
      POSTGRES_USER: gorm
      POSTGRES_PASSWORD: gorm
      POSTGRES_DB: gorm
    ports:
      - "9920:5432"

The default port for Postgres is 5432, so I mapped it to your 9920. Then, you can access the db container from the backend container by specifying:

db:9920