Building images in Compose in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the build context for the service.

Docker

services:
  web:
    build: [1]

Drag options to blanks, or click blank then click option'

A./app

Bimage: myapp

Cports: 80:80

Dvolumes: ./app:/app

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'image: myapp' under build instead of a path.

Confusing ports or volumes with build context.

The build key should point to the directory with the Dockerfile, which is the build context. Here, ./app is correct.

2fill in blank

medium

Complete the code to specify the Dockerfile name in the build section.

Docker

services:
  api:
    build:
      context: ./api
      dockerfile: [1]

Drag options to blanks, or click blank then click option'

Arequirements.txt

Bdocker-compose.yml

CDockerfile.api

Dapp.py

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated files like 'app.py' or 'requirements.txt' as Dockerfile.

Using the compose file name instead of Dockerfile.

The dockerfile key specifies the name of the Dockerfile to use. It must be a file name like Dockerfile.api.

3fill in blank

hard

Fix the error in the build section to correctly specify the build context and Dockerfile.

Docker

services:
  worker:
    build: [1]

Drag options to blanks, or click blank then click option'

A{ context: ./worker, dockerfile: Dockerfile.worker }

B./worker

Cports: 8080:80

Dimage: worker-image

Attempts:

3 left

💡 Hint

Common Mistakes

Using a simple string when multiple build options are needed.

Confusing image name with build context.

When specifying multiple build options, use a map with context and dockerfile keys inside build.

4fill in blank

hard

Fill both blanks to build an image with a custom Dockerfile and tag it.

Docker

services:
  app:
    build:
      context: [1]
      dockerfile: [2]
    image: myapp:latest

Drag options to blanks, or click blank then click option'

A./src

BDockerfile.custom

C./app

DDockerfile

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping context and dockerfile values.

Using default Dockerfile name when a custom one is needed.

The context is the folder path, here ./src. The dockerfile is the custom file name Dockerfile.custom.

5fill in blank

hard

Fill all three blanks to build an image with context, Dockerfile, and build args.

Docker

services:
  backend:
    build:
      context: [1]
      dockerfile: [2]
      args:
        ENV: [3]

Drag options to blanks, or click blank then click option'

A./backend

BDockerfile.prod

Cproduction

DDockerfile.dev

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong Dockerfile for production.

Confusing build args with environment variables.

The build context is ./backend, the Dockerfile is Dockerfile.prod, and the build argument ENV is set to production.

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