Pass ENV variables specified in shell from docker-compose to Dockerfile - docker-compose

Question

I would like to set the DEPLOY_ENV environment variable when I build my Docker image, but the value doesn't seem to be making it to the Dockerfile. Here's my build command:
docker-compose.yml
version: '3'
services:
web:
build: .
command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
environment:
- DEPLOY_ENV=${DEPLOY_ENV:-development}
ports:
- "3000:3000"
volumes:
- ".:/app"
When I run DEPLOY_ENV=staging docker-compose config, I can see that it replaces the DEPLOY_ENV variable as expected:
services:
web:
build:
context: /home/user/app
command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b
'0.0.0.0'"
environment:
DEPLOY_ENV: staging
ports:
- published: 3000
target: 3000
volumes:
- /home/user/app:/app:rw
version: '3'
However, I can't figure out how to populate the value in my Dockerfile. Here's a Dockerfile I'm using to test:
FROM ruby:2.7.2
SHELL ["/bin/bash", "-c"]
ENV RAILS_ENV=${DEPLOY_ENV}
ENV NODE_ENV=${DEPLOY_ENV}
ENV APP_HOME=/app
LABEL app=app
LABEL environment=${RAILS_ENV}
RUN echo ${DEPLOY_ENV}
When I run docker-compose build, it doesn't dipslay the value of DEPLOY_ENV:
Building web
[+] Building 1.4s (6/6) FINISHED
=> [internal] load build definition from Dockerfile 0.1s
=> => transferring dockerfile: 1.55kB 0.0s
=> [internal] load .dockerignore 0.1s
=> => transferring context: 35B 0.0s
=> [internal] load metadata for docker.io/library/ruby:2.7.2 1.0s
=> [1/2] FROM docker.io/library/ruby:2.7.2#sha256:1dd0106849233fcd913b7c4608078fa1a5sd5e3ce1af2a55e4d726b0d8868e2f 0.0s
=> CACHED [2/2] RUN echo ${DEPLOY_ENV} 0.0s
=> exporting to image 0.1s
=> => exporting layers 0.0s
=> => writing image sha256:27e159270471d3078fff8eb4eads7e4e586b345bfa13d3bdb3ec317266678549 0.0s
=> => naming to docker.io/library/app_web
I plan to set secret values when building my image and want to avoid hardcoding the values or use build args since they can be exposed.
What am I missing?

Answer 1

You could utilize the args primitive in docker compose, which passes build variables in your Dockerfile
args:
DEPLOY_ENV_ARG: ${DEPLOY_ENV:-development}
environment:
- DEPLOY_ENV=${DEPLOY_ENV:-development}
And in your Dockerfile
ARG DEPLOY_ENV_ARG
ENV RAILS_ENV=${DEPLOY_ENV_ARG}