docker .env file reuse declared variables

Question

When I run docker-compose build I have a .env file to be used for building environment variables on my docker-machine like this:

COMPOSE_PROJECT_NAME=radar
ENV=production
DB_NAME=postgres
DB_USER=postgres
DB_PASS=sho1c2ov3Phezaimah7eb2Tii4ohkah8k
DB_SERVICE=postgres
DB_PORT=5432
C_FORCE_ROOT="true"
PGHOST=postgres
PGDATABASE=postgres
PGUSER=postgres
PGPASSWORD=sho1c2ov3Phezaimah7eb2Tii4ohkah8k
PGPORT=5432

If you have noticed the redundancy like the 'DB_NAME' and 'PGDATABASE' is the same.. Is there a way to avoid this?

Answer 1

What about doing something like this in your Dockerfile?:

ARG DB_NAME
ENV DB_NAME=${DB_NAME} PGDATABASE=${DB_NAME} ...

Test

$ cat Dockerfile
FROM alpine:3.4

ARG DB_NAME
ENV DB_NAME=${DB_NAME} PGDATABASE=${DB_NAME}

RUN env

---

Compose File Version 2 or higher!

$ cat docker-compose.yml
version: '3' #'2' should work as well

services:
  docker-arg-env:
    build:
      context: .
      args:
        - DB_NAME
    command: env

---

$ cat .env
DB_NAME=world

---

$ docker-compose build
Building docker-arg-env
Step 1/4 : FROM alpine:3.4
 ---> 0766572b4bac
Step 2/4 : ARG DB_NAME
 ---> Running in a56be8426dd5
 ---> 4b1009ba9fad
Removing intermediate container a56be8426dd5
Step 3/4 : ENV DB_NAME ${DB_NAME} PGDATABASE ${DB_NAME}
 ---> Running in 5bbdd40e640e
 ---> 593105981a2a
Removing intermediate container 5bbdd40e640e
Step 4/4 : RUN env
 ---> Running in dadf204c7497
HOSTNAME=26ba10d264c2
SHLVL=1
HOME=/root
PGDATABASE=world
DB_NAME=world
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/
 ---> 9632ee1e0e37
Removing intermediate container dadf204c7497
Successfully built 9632ee1e0e37

---

$ docker-compose up
Creating network "dockerargenv_default" with the default driver
Creating dockerargenv_docker-arg-env_1
Attaching to dockerargenv_docker-arg-env_1
docker-arg-env_1  | PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
docker-arg-env_1  | HOSTNAME=5a6b51f4ecc7
docker-arg-env_1  | DB_NAME=world
docker-arg-env_1  | PGDATABASE=world
docker-arg-env_1  | HOME=/root

As you can see, PG_NAME and PGDATABASE are set during build-time and run-time.

Without Compose:

docker build --build-arg DB_NAME=world . and docker run docker-arg-env:latest env produce the same result.

Update

If you cannot (or don't want to) modify the Dockerfile try something like this:

$ cat .env
hello=world

---

$ cat docker-compose.yml
version: '2'
services:
  app:
    environment:
      - bar=${hello}
      - foo=${hello}
    image: someimage

---

$ docker-compose config
version: '2'
services:
  app:
    environment:
      - bar=world
      - foo=world
    image: someimage

See: https://docs.docker.com/compose/environment-variables/#the-env-file