Using env variables from docker compose in a Dockerfile

Question

I am confused, because it seems that an environment variable which is set in docker-compose.yml causes docker to rebuild my image on docker-compose up:

Dockerfile for apache_php_with_custom_docroot

FROM php:7.1.11-apache-jessie

# Set apache document root
ENV APACHE_DOCUMENT_ROOT /var/www/web
RUN sed -ri -e 's!/var/www/html!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/sites-available/*.conf
RUN sed -ri -e 's!/var/www/!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/apache2.conf /etc/apache2/conf-available/*.conf

I'm building it with docker build -t apache_php_with_custom_docroot .

docker-compose.yml

version: "3"
services:
  web:    
    image: apache_php_with_custom_docroot
    environment:
      - APACHE_DOCUMENT_ROOT=/var/www/anotherfolder

So, the strange thing to me is: It works to set the APACHE_DOCUMENT_ROOT. When I run docker-compose up -d apache uses the docroot /var/www/anotherfolder.

I was sure, when my image was build, then I can't change it in my docker-compose file. I read something about build parameters e.g. here: Docker-compose: Set a variable in env file and use it in Dockerfile

But I did not set build params. How does docker know, that he has to rebuild my image? Where can I read about the principles behind this?

a bit rusty with docker! , as far as i know , every change in docker-compose.yml is need to make an up (because docker reads it and create the virtual filesystem and the container behavior), so i thing you need to attach this on the entrypoint , where you i.e can read a file with vars and load it every time containers start ... — Joaquin Javi
– Joaquin Javi, Commented Nov 7, 2017 at 15:23

Joaquin Javi · Accepted Answer · 2017-11-07 15:33:40Z

-1

define a entry point in your dockerfile like:

ENTRYPOINT bash $APACHE_DOCUMENT_ROOT/entrypoint.sh

And then in your entry point every time containers starts, you will be able to read mapped volume with a file and inject in your container or declare the environments or related startup task you will need

enviroment=$(cat /mapped_config/enviroment.env)
if grep -Fxq "$enviroment" /home/.bashrc
  then 
      echo "Environment setted propertly"
  else
      echo "$enviroment" >> /home/.bashrc
      echo "enviorment setted!"
fi

It's just an idea , and probably not the best way , is how i deal it in past.

answered Nov 7, 2017 at 15:33

Joaquin Javi

8867 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1383029 Over a year ago

-1, because this is not an answer to my question. I do not understand why my code works, i did not search for an answer how this could be done in another way...

Joaquin Javi Over a year ago

i just trying to explain when you do docker-compose up, docker will read your docker-compose.yml file and build your image ( with config and vars you defined ), after if you will make any change on docker-compose file ( such as change env var etc..) you will need to build again ( not stop , start or restart ) if you wanna read something dynamically when containers starts you can do this staff in the entrypoint

Collectives™ on Stack Overflow

Using env variables from docker compose in a Dockerfile

1 Answer 1

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Linked

Related