.env is used within docker-compose.yaml to load template variables. It follows the format of <template-variable>=<value>:

CONCIERGE_PORT=9090
A_M_PORT=6060
N_Z_PORT=7070
OTHERS_PORT=8080
SERVER_PORT=9876  

We can view the docker-compose.yaml with substituted values by running the following command:

$ pwd 
GO-WORKSPACE/src/github.com/last-ent/distributed-go/chapter8/goophr 
$ docker-compose config 
services: 
  a_m_librarian: 
    build: 
      context: /home/entux/Documents/Code/GO-WORKSPACE/src/github.com/last-ent/distributed-go/chapter8/goophr/librarian 
    environment: 
      API_PORT: '6060' 
    ports: 
    - 6060:6060/tcp 
  concierge: 
    build: 
      context: /home/entux/Documents/Code/GO-WORKSPACE/src/github.com/last-ent/distributed-go/chapter8/goophr/concierge 
    environment: 
      API_PORT: '9090' 
      LIB_A_M: http://a_m_librarian:6060/api 
      LIB_N_Z: http://n_z_librarian:7070/api 
      LIB_OTHERS: http://others_librarian:8080/api 
    links: 
    - a_m_librarian 
    - n_z_librarian 
    - others_librarian 
    - file_server 
    ports: 
    - 9090:9090/tcp 
  file_server: 
    build: 
      context: /home/entux/Documents/Code/GO-WORKSPACE/src/github.com/last-ent/distributed-go/chapter8/goophr/simple-server 
    ports: 
    - 9876:9876/tcp 
  n_z_librarian: 
    build: 
      context: /home/entux/Documents/Code/GO-WORKSPACE/src/github.com/last-ent/distributed-go/chapter8/goophr/librarian 
    environment: 
      API_PORT: '7070' 
    ports: 
    - 7070:7070/tcp 
  others_librarian: 
    build: 
      context: /home/entux/Documents/Code/GO-WORKSPACE/src/github.com/last-ent/distributed-go/chapter8/goophr/librarian 
    environment: 
      API_PORT: '8080' 
    ports: 
    - 8080:8080/tcp 
version: '3.0'