Building images in Compose in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker-compose build with context path

Given this docker-compose.yml snippet, what will be the output when running docker-compose build?

Docker

services:
  web:
    build:
      context: ./app
      dockerfile: Dockerfile.web
    image: mywebapp:latest

A Successfully built image 'mywebapp:latest' using Dockerfile.web from ./app directory

B Error: Cannot find Dockerfile in the root directory

C Build completed but image tagged as 'latest' without 'mywebapp' prefix

D Build fails due to missing context path

💡 Hint

The build context and dockerfile path determine where Docker looks for files.

The build context './app' tells Docker to use that directory. The dockerfile 'Dockerfile.web' is found there, so the image builds successfully and is tagged as 'mywebapp:latest'.

❓ Configuration

intermediate

Correct syntax for build args in Compose

Which docker-compose.yml snippet correctly passes build arguments to the Docker build process?

A

build:
  context: .
  args:
    APP_ENV: production
    DEBUG: 'false'

B

build:
  context: .
  build_args:
    APP_ENV: production
    DEBUG: 'false'

C

build:
  context: .
  args:
    - APP_ENV=production
    - DEBUG=false

D

build:
  context: .
  build-args:
    APP_ENV: production
    DEBUG: 'false'

💡 Hint

Check the official Compose syntax for build arguments.

The correct key is 'args' under 'build'. It accepts a map of argument names and values. Other keys like 'build_args' or 'build-args' are invalid.

❓ Troubleshoot

advanced

Diagnosing build failure due to missing Dockerfile

You run docker-compose build but get the error: "Cannot locate specified Dockerfile: Dockerfile.prod". Your docker-compose.yml contains:

services:
  api:
    build:
      context: ./backend
      dockerfile: Dockerfile.prod

What is the most likely cause?

A The dockerfile key must be an absolute path, not relative

B The context path ./backend is incorrect and should be ./api

C Dockerfile.prod does not exist inside the ./backend directory

D Docker Compose requires the Dockerfile to be named exactly 'Dockerfile'

💡 Hint

Check the location of the Dockerfile relative to the context.

The dockerfile path is relative to the build context. If Dockerfile.prod is missing inside ./backend, the build fails with this error.

🔀 Workflow

advanced

Order of steps in multi-stage build with Compose

In a multi-stage Docker build defined in a Dockerfile, which step happens first when running docker-compose build?

A Docker builds only the stages referenced by the final image

B Only the final stage is built, skipping earlier stages

C Docker builds all stages in parallel

D All stages are built sequentially from the first to the last stage

💡 Hint

Think about how Docker processes multi-stage builds.

Docker builds each stage in order because later stages can depend on earlier ones. The final image uses the last stage, but all stages are built sequentially.

✅ Best Practice

expert

Optimizing build cache usage in Compose builds

Which practice best helps Docker Compose reuse build cache effectively when building images?

A Use COPY commands with wildcards to include all files in one step

B Keep the build context minimal and avoid changing files unnecessarily

C Add a random build argument to force cache invalidation each time

D Always use the latest tag for base images to get fresh builds

💡 Hint

Think about what invalidates Docker cache during build.

Docker cache is invalidated if files in the build context change. Keeping context small and stable helps reuse cache. Using latest tags or random args forces rebuilds. COPY with wildcards can cause unnecessary cache misses.