Pass environment variables from a .env (dotenv) file into your Docker containers using docker-compose and build ARGs.

docker_compose_dotenv_environment_variables

# .env
ANIMAL=dog
COLOUR=brown

# docker-compose.yml
...
services:
  web:
	...
	build:
      context: .
      args:
        - ANIMAL=$ANIMAL
        - COLOUR=$COLOUR
...

# Dockerfile
...
ARG ANIMAL=dog
ARG COLOUR=brown
ENV ANIMAL=$ANIMAL
ENV COLOUR=$COLOUR
...

# container
> printenv
...
ANIMAL=dog
COLOUR=brown
...