GitHub Actions with Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker build in GitHub Actions

You have this GitHub Actions step to build a Docker image:

      - name: Build Docker Image
        run: docker build -t myapp:latest .

What will be the output if the Dockerfile is missing in the repository root?

A Error: Cannot locate Dockerfile: No such file or directory

B Successfully builds the image with default settings

C Build completes but image is empty

D Warning: Dockerfile not found, using default base image

💡 Hint

Docker build requires a Dockerfile in the specified context directory.

Docker build command looks for a Dockerfile in the current directory by default. If missing, it throws an error indicating it cannot find the Dockerfile.

🔀 Workflow

intermediate

Correct GitHub Actions step to push Docker image

Which GitHub Actions step correctly logs in to Docker Hub and pushes an image named 'myapp:latest'?

A

- name: Push Docker Image
  run: |
    docker login -u ${{ secrets.DOCKER_USER }} -p ${{ secrets.DOCKER_PASS }}
    docker push myapp:latest

B

- name: Push Docker Image
  run: |
    docker push myapp:latest
    docker login -u ${{ secrets.DOCKER_USER }} -p ${{ secrets.DOCKER_PASS }}

C

- name: Push Docker Image
  run: |
    docker login ${{ secrets.DOCKER_USER }} ${{ secrets.DOCKER_PASS }}
    docker push myapp:latest

D

- name: Push Docker Image
  run: |
    docker push myapp:latest

💡 Hint

Docker login must happen before pushing the image.

You must login to Docker Hub before pushing images. The login command requires username and password flags. The order matters: login first, then push.

❓ Configuration

advanced

GitHub Actions Docker cache usage

You want to speed up Docker builds in GitHub Actions by caching layers. Which configuration snippet correctly uses the official Docker cache action?

A

- name: Cache Docker layers
  uses: docker/build-push-action@v4
  with:
    cache-from: type=local,src=/tmp/.buildx-cache
    cache-to: type=local,dest=/tmp/.buildx-cache

B

- name: Cache Docker layers
  uses: actions/cache@v3
  with:
    path: /tmp/.buildx-cache
    key: ${{ runner.os }}-buildx-${{ hashFiles('**/Dockerfile') }}

C

- name: Cache Docker layers
  uses: actions/cache@v3
  with:
    path: /var/lib/docker
    key: ${{ runner.os }}-docker-${{ hashFiles('**/Dockerfile') }}

D

- name: Cache Docker layers
  uses: actions/cache@v3
  with:
    path: /tmp/.buildx-cache
    key: ${{ runner.os }}-buildx-${{ github.sha }}

💡 Hint

Cache key should depend on Dockerfile content hash for cache invalidation.

Caching Docker layers requires a cache path and a key that changes when Dockerfile changes. Option B uses the correct path and a key based on Dockerfile hash.

❓ Troubleshoot

advanced

Diagnosing Docker build failure in GitHub Actions

A GitHub Actions workflow fails with this error during docker build:

failed to solve with frontend dockerfile.v0: failed to read dockerfile: open /github/workspace/Dockerfile: no such file or directory

What is the most likely cause?

A The docker build command is missing the -t tag option

B Docker daemon is not running on the GitHub runner

C The Dockerfile is named incorrectly or missing in the repository root

D The GitHub Actions runner lacks permissions to run docker commands

💡 Hint

The error says it cannot open the Dockerfile file.

The error indicates the Dockerfile is not found in the expected location. This usually means the file is missing or named differently.

✅ Best Practice

expert

Optimal multi-stage Docker build in GitHub Actions

You want to build a multi-stage Docker image in GitHub Actions to reduce final image size. Which Dockerfile snippet correctly implements a two-stage build for a Go application?

A

FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine:latest
COPY myapp /usr/local/bin/myapp
CMD ["myapp"]

B

FROM alpine:latest AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM golang:1.20
COPY --from=builder /app/myapp /usr/local/bin/myapp
CMD ["myapp"]

C

FROM golang:1.20
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine:latest
COPY /app/myapp /usr/local/bin/myapp
CMD ["myapp"]

D

FROM golang:1.20 AS builder
WORKDIR /app
COPY . .
RUN go build -o myapp
FROM alpine:latest
COPY --from=builder /app/myapp /usr/local/bin/myapp
CMD ["myapp"]

💡 Hint

The final stage should copy the binary from the builder stage using --from.

Multi-stage builds use named stages. The final stage copies the built binary from the builder stage using COPY --from=builder. Option D correctly implements this.

Practice

1. What is the main purpose of using GitHub Actions with Docker in a project?

easy

A. To automate building and testing Docker images on code changes

B. To manually run Docker commands on your local machine

C. To replace Docker with a different container tool

D. To write Dockerfiles without using any commands

Solution

1. Step 1: Understand GitHub Actions role

   GitHub Actions automates tasks like building and testing when code changes happen.

2. Step 2: Connect automation with Docker

   Using GitHub Actions with Docker means automating Docker image builds and tests on events like pushes.

3. Final Answer:

   To automate building and testing Docker images on code changes -> Option A

4. Quick Check:

   Automation with Docker = D [OK]

Hint: GitHub Actions automate tasks triggered by code events [OK]

Common Mistakes:

• Thinking GitHub Actions replace Docker itself
• Confusing manual commands with automation
• Assuming Dockerfiles are written by GitHub Actions

2. Which of the following is the correct syntax to build a Docker image inside a GitHub Actions workflow step?

easy

A. run: docker start -t myimage .

B. run: docker create myimage .

C. run: docker push -t myimage .

D. run: docker build -t myimage .

Solution

1. Step 1: Recall Docker build command

   The command to build an image is 'docker build' with '-t' to tag the image.

2. Step 2: Match with GitHub Actions step syntax

   In GitHub Actions, 'run:' is used to run shell commands, so 'run: docker build -t myimage .' is correct.

3. Final Answer:

   run: docker build -t myimage . -> Option D

4. Quick Check:

   Build image command = C [OK]

Hint: Use 'docker build -t name .' to build images [OK]

Common Mistakes:

• Using 'docker create' instead of 'docker build'
• Confusing 'docker start' with building images
• Using 'docker push' to build images

3. Given this GitHub Actions workflow step:

run: |
  docker build -t test-image .
  docker run --rm test-image echo Hello

What will be the output when this step runs successfully?

medium

A. test-image

B. Hello

C. docker build completed

D. Error: image not found

Solution

1. Step 1: Understand the commands run

   The first command builds an image tagged 'test-image'. The second runs a container from it that prints 'Hello'.

2. Step 2: Identify the output of the run command

   The 'docker run' command runs 'echo Hello' inside the container, so the output is 'Hello'.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   docker run echo output = Hello [OK]

Hint: Run command output is what container prints [OK]

Common Mistakes:

• Thinking build command outputs 'test-image'
• Expecting build success message as output
• Assuming image not found error without build

4. You have this GitHub Actions step:

run: docker build -t myapp .
run: docker run myapp

The workflow fails with 'Error: Dockerfile not found'. What is the likely cause?

medium

A. The Dockerfile is missing in the repository

B. The two 'run' commands are separate steps and do not share state

C. The image name 'myapp' is invalid

D. The 'docker run' command syntax is incorrect

Solution

1. Step 1: Understand docker build requirement

   The 'docker build .' command requires a Dockerfile in the current directory (repo root after checkout).

2. Step 2: Identify the error cause

   If the Dockerfile is missing from the repository, the build step fails with 'Dockerfile not found' error.

3. Final Answer:

   The Dockerfile is missing in the repository -> Option A

4. Quick Check:

   docker build fails = no Dockerfile [OK]

Hint: Ensure Dockerfile exists in repository root [OK]

Common Mistakes:

• Thinking separate steps prevent image sharing
• Assuming image name 'myapp' is invalid
• Believing 'docker run' syntax causes the error

5. You want to create a GitHub Actions workflow that builds a Docker image only when a push happens to the 'main' branch and then pushes the image to Docker Hub. Which of these YAML snippets correctly sets the trigger and steps?

hard

A. on:
push:
tags:
- main
jobs:
build:
runs-on: ubuntu-latest
steps:
- run: docker build -t myuser/myapp .
- run: docker push myuser/myapp

B. on:
pull_request:
branches:
- main
jobs:
build:
runs-on: ubuntu-latest
steps:
- run: docker build -t myapp .
- run: docker push myapp

C. on:
push:
branches:
- main
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v3
- run: docker build -t myuser/myapp:latest .
- run: echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin
- run: docker push myuser/myapp:latest

D. on:
push:
branches:
- develop
jobs:
build:
runs-on: ubuntu-latest
steps:
- run: docker build -t myuser/myapp .
- run: docker push myuser/myapp

Solution

1. Step 1: Check trigger event and branch

   on: push: branches: - main jobs: build: runs-on: ubuntu-latest steps: - uses: actions/checkout@v3 - run: docker build -t myuser/myapp:latest . - run: echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin - run: docker push myuser/myapp:latest triggers on push to 'main' branch, which matches the requirement.

2. Step 2: Verify build and push steps

   on: push: branches: - main jobs: build: runs-on: ubuntu-latest steps: - uses: actions/checkout@v3 - run: docker build -t myuser/myapp:latest . - run: echo ${{ secrets.DOCKER_PASSWORD }} | docker login -u ${{ secrets.DOCKER_USERNAME }} --password-stdin - run: docker push myuser/myapp:latest includes checkout, build with proper tagging, login using secrets, and push commands correctly.

3. Final Answer:

   The snippet that triggers on push to main, checks out code, logs in with secrets, builds tagged image, and pushes -> Option C

4. Quick Check:

   Push on main + login + build + push = A [OK]

Hint: Use 'on: push: branches: - main' and login before push [OK]

Common Mistakes:

• Using pull_request instead of push event
• Triggering on tags instead of branches
• Pushing without Docker login step