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Using-Docker-Compose-overrides.md

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Using Docker Compose overrides

Sometimes, we want to run our applications in different environments that need specific configuration settings. Docker Compose provides a handy capability to address exactly this issue.

Let's make a specific sample. We can define a base Docker Compose file and then define environment-specific overrides. Let's assume we have a file called docker-compose.base.yml with the following content:

Navigate to :

cd c:\DJK\Lab-10-Docker-Compose\sample\ci-project\

version: "2.4"
services:
  web:
    image: fredysa/web:1.0
  db:
    image: fredysa/db:1.0
    volumes:
      - pets-data:/var/lib/postgresql/data

volumes:
  pets-data:

This only defines the part that should be the same in all environments. All specific settings have been taken out.

Let's assume for a moment that we want to run our sample application on a CI system, but there we want to use different settings for the database. The Dockerfile we used to create the database image looked like this:

FROM postgres:12.0-alpine
COPY init-db.sql /docker-entrypoint-initdb.d/
ENV POSTGRES_USER dockeruser
ENV POSTGRES_PASSWORD dockerpass
ENV POSTGRES_DB pets

Notice the three environment variables we define on lines 3 through 5. The Dockerfile of the web service has similar definitions. Let's say that on the CI system, we want to do the following:

Build the images from code

Define POSTGRES_PASSWORD as ci-pass Map container port 3000 of the web service to host port 5000 Then, the corresponding override file would look like this:

version: "2.4"
services:
  web:
    build: web
    ports:
      - 5000:3000
    environment:
      POSTGRES_PASSWORD: cipass
  db:
    build: db
    environment:
      POSTGRES_PASSWORD: cipass

And we can run this application with the following command:

$ docker-compose -f docker-compose.yml -f docker-compose-ci.yml up -d --build

Note that with the first -f parameter, we provide the base Docker Compose file, and with the second one, we provide the override. The --build parameter is used to force docker-compose to rebuild the images.

Note: When using environment variables, note the following precedence:

  • Declaring them in the Docker file defines a default value
  • Declaring the same variable in the Docker Compose file overrides the value from the Dockerfile

Had we followed the standard naming convention and called the base file just docker-compose.yml and the override file docker-compose.override.yml instead, then we could have started the application with docker-compose up -dwithout explicitly naming the compose files.