Building images in Compose in Docker - Commands & Configuration

Introduction

Sometimes you want to create a custom container image for your app instead of using a ready-made one. Docker Compose lets you build these images automatically from a simple file, so you can run your app with all its settings in one step.

When you have your own app code and want to package it with its environment into a container.

When you want to share your app setup with teammates without manual steps.

When you need to customize an existing image by adding files or changing settings.

When you want to run multiple containers that depend on your custom-built image.

When you want to automate building and running your app with one command.

Config File - docker-compose.yml

docker-compose.yml

version: '3.9'
services:
  webapp:
    build:
      context: ./app
      dockerfile: Dockerfile
    ports:
      - "8080:80"

version: specifies the Compose file format version.

services: defines the containers to run.

webapp: is the service name.

build: tells Compose to build an image from the Dockerfile in the ./app folder.

ports: maps port 8080 on your computer to port 80 in the container.

Commands

This command builds the Docker image for the service defined in docker-compose.yml using the Dockerfile in the specified context folder.

Terminal

docker-compose build

Expected Output

Building webapp

Step 1/4 : FROM nginx:alpine

 ---> 3f53bb00af94

Step 2/4 : COPY . /usr/share/nginx/html

 ---> Using cache

 ---> 7d9495d03763

Step 3/4 : EXPOSE 80

 ---> Using cache

 ---> 8c2e066076a5

Step 4/4 : CMD ["nginx", "-g", "daemon off;"]

 ---> Using cache

 ---> 5d0da3dc9764

Successfully built 5d0da3dc9764

Successfully tagged webapp_webapp:latest

This command starts the container using the image just built and runs it in the foreground so you can see its output.

Terminal

docker-compose up

Expected Output

Creating network "webapp_default" with the default driver

Creating webapp_webapp_1 ... done

Attaching to webapp_webapp_1

webapp_webapp_1  | 172.17.0.1 - - [date] "GET / HTTP/1.1" 200 612 "-" "curl/7.64.1"

This command shows the status of the containers started by Compose, so you can check if your app is running.

Terminal

docker-compose ps

Expected Output

    Name                  Command               State           Ports         

----------------------------------------------------------------------------

webapp_webapp_1   nginx -g 'daemon off;'      Up      0.0.0.0:8080->80/tcp

This command stops and removes the containers and network created by docker-compose up, cleaning up your environment.

Terminal

docker-compose down

Expected Output

Stopping webapp_webapp_1 ... done

Removing webapp_webapp_1 ... done

Removing network webapp_default

Key Concept

If you remember nothing else from this pattern, remember: Docker Compose can build your app's image from a Dockerfile automatically, so you don't have to build it manually before running.

Common Mistakes

Not specifying the build context or Dockerfile path correctly in docker-compose.yml

Docker Compose won't find the Dockerfile and fails to build the image.

Set the build context to the folder containing your Dockerfile and specify the Dockerfile name if it's not the default.

Running docker-compose up without building first when the image is not built

The container fails to start because the image does not exist.

Run docker-compose build first or use docker-compose up --build to build and start in one step.

Not mapping ports in docker-compose.yml when your app listens on a port

You cannot access your app from outside the container.

Add a ports section to map container ports to your machine's ports.

Summary

Define the build context and Dockerfile in docker-compose.yml under the build key.

Use docker-compose build to create the image from your Dockerfile.

Use docker-compose up to start the container from the built image.

Use docker-compose ps to check running containers and docker-compose down to stop and clean up.