Building images in Compose in Docker - Mini Project: Build & Apply

Building images in Compose

📖 Scenario: You are working on a small web project that uses Docker Compose to manage services. You want to build a custom Docker image for your web service using a Dockerfile.

🎯 Goal: Learn how to define a Docker Compose file that builds a Docker image from a Dockerfile and runs a container from that image.

📋 What You'll Learn

Create a Dockerfile with a simple base image

Create a docker-compose.yml file that builds the image from the Dockerfile

Define a service in Compose that uses the built image

Run the Compose setup and verify the container runs

💡 Why This Matters

🌍 Real World

Building images in Compose is common when you want to customize your app environment and easily run multiple services together.

💼 Career

DevOps engineers and developers use Docker Compose to manage multi-container applications and automate image builds.

1

Create a Dockerfile

Create a file named Dockerfile with these exact contents:
FROM alpine:latest
CMD ["echo", "Hello from custom image"]

Docker

# Create a Dockerfile with FROM alpine:latest and CMD ["echo", "Hello from custom image"]

Hint

The Dockerfile must start with FROM alpine:latest and end with CMD ["echo", "Hello from custom image"].

2

Create docker-compose.yml with build config

Create a file named docker-compose.yml with a service called web that builds the image from the current directory using the build key.

Docker

FROM alpine:latest
CMD ["echo", "Hello from custom image"]

# Create docker-compose.yml with service 'web' that builds from current directory
# Your code here

Hint

The Compose file must have services: with a web: service that uses build: . to build from the current folder.

3

Add ports mapping to the service

In the docker-compose.yml file, add a ports section under the web service that maps port 8080 on the host to port 80 in the container.

Docker

FROM alpine:latest
CMD ["echo", "Hello from custom image"]

version: '3.8'
services:
  web:
    build: .
    # Add ports mapping 8080:80 below
    # Your code here

Hint

Indent the ports: section under web: and add - "8080:80" to map ports.

4

Run docker-compose and show container output

Run docker-compose up --build --abort-on-container-exit in the terminal to build the image and start the container. Then, write a command to display the container logs using docker-compose logs web.

Docker

FROM alpine:latest
CMD ["echo", "Hello from custom image"]

version: '3.8'
services:
  web:
    build: .
    ports:
      - "8080:80"

# Run docker-compose up with build and abort on exit
# Then show logs of web service
# Your code here

Hint

Use docker-compose up --build --abort-on-container-exit to build and run, then docker-compose logs web to see the output.

Practice

1. What does the build key do in a Docker Compose file?

easy

A. It sets the network mode for the container.

B. It runs a container from an existing image.

C. It specifies environment variables for the container.

D. It tells Compose how to build a Docker image from a directory or Dockerfile.

Solution

1. Step 1: Understand the role of build in Compose

   The build key tells Docker Compose to create an image from source files, usually by specifying a directory with a Dockerfile.

2. Step 2: Compare with other keys

   Other keys like image specify existing images, and environment sets variables, so they do not build images.

3. Final Answer:

   It tells Compose how to build a Docker image from a directory or Dockerfile. -> Option D

4. Quick Check:

   build = build image [OK]

Hint: Remember: build = create image from source [OK]

Common Mistakes:

• Confusing build with image key
• Thinking build runs containers
• Mixing build with environment variables

2. Which of the following is the correct syntax to specify a custom Dockerfile in a Compose service?

easy

A. build: ./app
dockerfile: Dockerfile.custom

B. build:
path: ./app
file: Dockerfile.custom

C. build:
context: ./app
dockerfile: Dockerfile.custom

D. build:
source: ./app
file: Dockerfile.custom

Solution

1. Step 1: Recall correct keys under build

   The correct keys are context for directory and dockerfile for custom Dockerfile name.

2. Step 2: Check each option

   build: context: ./app dockerfile: Dockerfile.custom uses context and dockerfile correctly. Others use invalid keys like path, file, or source.

3. Final Answer:

   build: context: ./app dockerfile: Dockerfile.custom -> Option C

4. Quick Check:

   Use context and dockerfile keys [OK]

Hint: Use 'context' and 'dockerfile' keys exactly [OK]

Common Mistakes:

• Using 'path' or 'file' instead of 'context' and 'dockerfile'
• Indentation errors under build
• Putting dockerfile outside build block

3. Given this Compose snippet:

services:
  web:
    build:
      context: ./webapp
    image: mywebapp:latest

What happens when you run docker compose up --build?

medium

A. It builds the image from ./webapp and tags it as mywebapp:latest, then starts the container.

B. It pulls the image mywebapp:latest from Docker Hub and runs it.

C. It builds the image but does not tag it as mywebapp:latest.

D. It starts the container without building or pulling any image.

Solution

1. Step 1: Understand build and image keys together

   The build key builds an image from the given context. The image key tags the built image with the given name.

2. Step 2: Effect of --build flag

   The --build flag forces Compose to build the image before starting containers.

3. Final Answer:

   It builds the image from ./webapp and tags it as mywebapp:latest, then starts the container. -> Option A

4. Quick Check:

   build + image = build and tag image [OK]

Hint: build + image tags image before running [OK]

Common Mistakes:

• Thinking image pulls from Docker Hub automatically
• Assuming build does not tag image
• Ignoring --build flag effect

4. You wrote this Compose service:

services:
  api:
    build: ./api
    dockerfile: Dockerfile.api

When running docker compose build, you get an error. What is wrong?

medium

A. You must specify context instead of build.

B. The dockerfile key must be inside the build block, not at the same level.

C. The build key cannot be a string; it must be a dictionary.

D. Docker Compose does not support custom Dockerfile names.

Solution

1. Step 1: Check Compose syntax for build and dockerfile

   The dockerfile key must be nested inside the build dictionary, not at the same level as build.

2. Step 2: Identify the error cause

   Here, build is a string, and dockerfile is separate, causing Compose to fail parsing.

3. Final Answer:

   The dockerfile key must be inside the build block, not at the same level. -> Option B

4. Quick Check:

   dockerfile inside build block [OK]

Hint: Put dockerfile under build, not beside it [OK]

Common Mistakes:

• Placing dockerfile outside build block
• Using build as string with dockerfile key separately
• Assuming dockerfile key is global

5. You want to build two services in Compose, frontend and backend, each with different Dockerfiles in the same directory ./app. How do you configure the build section for both services correctly?

hard

A. Use context: ./app for both, and specify different dockerfile names under each service's build.

B. Use different context paths for each service pointing to the same directory.

C. Use one service with build and the other with image only.

D. You cannot build two images from the same directory with different Dockerfiles.

Solution

1. Step 1: Understand build context and dockerfile usage

   You can use the same context directory but specify different dockerfile names to build different images from the same folder.

2. Step 2: Check options for multiple services

   Use context: ./app for both, and specify different dockerfile names under each service's build. correctly uses the same context with different Dockerfiles. Use different context paths for each service pointing to the same directory. is redundant, C limits building, and D is false.

3. Final Answer:

   Use context: ./app for both, and specify different dockerfile names under each service's build. -> Option A

4. Quick Check:

   Same context + different dockerfile names [OK]

Hint: Same folder, different dockerfile names per service [OK]

Common Mistakes:

• Trying to use different contexts for same folder
• Thinking multiple Dockerfiles in one folder is unsupported
• Not specifying dockerfile key for each service