Learn the differences between ENVs and ARGs in Docker

01.Dockerfile

FROM alpine:latest

# 01- ENV set directly in Dockerfile.
# Available during BUILDTIME and RUNTIME.
ENV LOCAL_ENV="hi I am LOCAL_ENV"
RUN echo $LOCAL_ENV

# 02 - ARG sent via command (docker or docker-compose)
# Available during BUILDTIME only.
ARG BUILDTIME_ENV
RUN echo $BUILDTIME_ENV

COPY script.sh ./script.sh

CMD ./script.sh

02.docker-compose.yml

version: "2"
services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
      # 03 - ARG injected into _building_ container.
      # Available during BUILDTIME only.
      args:
        - BUILDTIME_ENV="I am buildtime env"

    environment:
      # 04 - ENV Injected into _running_ container.
      # Available during RUNTIME only.
      - RUNTIME_ENV="I am runtime env"

03.script.sh

#!/bin/ash
echo $LOCAL_ENV $BUILDTIME_ENV $RUNTIME_ENV

Use the following commands to build and run:

docker build -t orlando-elixir .
docker run --rm orlando-elixir

docker build --build-arg BUILDTIME_ENV="hi I am buildtime env" -t orlando-elixir .

is the same as

docker-compose build

and finally

docker run --env RUNTIME_ENV="I am runtime env" --rm orlando-elixir