docker - How to load several environment variables without cluttering the Dockerfile?

Question

I am working on a simple Docker image that has a large number of environment variables. Are you able to import an environment variable file like with docker-compose? I cannot find anything about this in the docker file documentation.

Dockerfile

FROM python:3.6

ENV ENV1 9.3
ENV ENV2 9.3.4
...

ADD . /

RUN pip install -r requirements.txt

CMD [ "python", "./manager.py" ]

I guess a good way to rephrase the question would be: how do you efficiently load multiple environment variables in a Dockerfile? If you are not able to load a file, you would not be able to commit a docker file to GitHub.

question from:https://stackoverflow.com/questions/46917831/how-to-load-several-environment-variables-without-cluttering-the-dockerfile

Answer 1

Yes, there are a couple of ways you can do this.

Docker Compose

In Docker Compose, you can supply environment variables in the file itself, or point to an external env file:

# docker-compose.yml
version: '2'
services:

  service-name:
    image: service-app
    environment:
    - GREETING=hello
    env_file:
    - .env

Incidentally, one nice feature that is somewhat related is that you can use multiple Compose files, with each subsequent one adding to the other. So if the above were to define a base, you can then do this (e.g. per run-time environment):

# docker-compose-dev.yml
version: '2'
services:

  service-name:
    environment:
    - GREETING=goodbye

You can then run it thus:

docker-compose -f docker-compose.yml -f docker-compose-dev.yml up

Docker only

To do this in Docker only, use your entrypoint or command to run an intermediate script, thus:

#Dockerfile

....

ENTRYPOINT ["sh", "bin/start.sh"]

And then in your start script:

#!/bin/sh

source .env

python /manager.py

I've used this related answer as a helpful reference for myself in the past.

Update on PID 1

To amplify my remark in the comments, if you make your entry point a shell or Python script, it is likely that Unix signals (stop, kill, etc) will not be passed onto your process. This is because that script will become process ID 1, which makes it the parent process of all other processes in the container - in Linux/Unix there is an expectation that this PID will forward signals to its children, but unless you explicitly implement that, it won't happen.

To rectify this, you can install an init system. I use dumb-init from Yelp. This repo also features plenty of detail if you want to understand it a bit better, or simple install instructions if you just want to "install and forget".