How to copy file contents to a service during docker-compose up? - docker

Question

I have 2 files:
.env
docker-compose.yml
docker-compose.yml looks like this:
version: '3'
services:
database:
image: mysql:5.7
myapp:
image: me/some-image
depends_on:
- database
env_file: .env
myapp is a web service app that needs a .env file or optionally it can access the environment variables if no .env file is present.
As of now, the myapp is accessing the environment variables because I don't want the .env file to be included in the image build for security reasons. What I did is to pass a env_file: .env to the myapp service in the docker-compose.yml file so it will rely to the environment variables of the service instead of a .env file.
Now, I really want to add a .env file to the myapp service when running docker-compose up. Take note that the myapp web service will throw an error if it didnt find a .env file and the option is to look for a .env file instead of getting from the environment variables of the container.
Is there a way to create a .env file when running docker-compose up and copy the contents of the .env file on the host? Thank you in advance.

Answer 1

You can use bind mount to mount file into the container
change the target location to the one your app requires
version: '3'
services:
database:
image: mysql:5.7
myapp:
image: me/some-image
depends_on:
- database
env_file: .env
volumes:
- type: bind
source: ./.env
target: /envfile/.env
readonly: true

Answer 2

You shouldn't be storing the environment variables in the container.
According to TwelveFactor-
The twelve-factor app stores config in environment variables (often shortened to env vars or env). Env vars are easy to change between deploys without changing any code; unlike config files, there is little chance of them being checked into the code repo accidentally; and unlike custom config files, or other config mechanisms such as Java System Properties, they are a language- and OS-agnostic standard.
You are correctly passing the env file. The env file needs to be in the format of <key>=<value>.
For example -
DB_USERNAME=user
DB_PASSWORD=password
DB_SCHEMA=db
DB_PORT=3306
DB_HOST=db
REDIS_HOST=redis
REDIS_PORT=6379
REDIS_DB=0
In your application, you don't need to care about if the .env file is present, you simply read the environment variable.
Example in python -
import os
db_username = os.environ['DB_USERNAME']
You should not be coupling your environment variable to a file. It should read from the runtime environment.