GitHub Actions with Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is GitHub Actions?

GitHub Actions is a tool that helps automate tasks like testing and deploying code when you push changes to your GitHub repository.

beginner

How does Docker fit into GitHub Actions?

Docker lets you package your app and its environment into a container. GitHub Actions can build and run these Docker containers automatically during workflows.

beginner

What is a workflow file in GitHub Actions?

A workflow file is a YAML file in your repo that tells GitHub Actions what steps to run and when, like building a Docker image after code changes.

intermediate

What does the 'docker/build-push-action' do in a GitHub Actions workflow?

It builds a Docker image from your code and can push it to a container registry like Docker Hub automatically.

beginner

Why use GitHub Actions with Docker instead of manual builds?

It saves time by automating builds and tests, ensures consistency, and helps catch errors early before deploying your app.

What file format is used to define GitHub Actions workflows?

A XML

B JSON

C TXT

D YAML

GitHub Actions workflows are defined using YAML files, which are easy to read and write.

Which GitHub Actions step is commonly used to build and push Docker images?

A actions/checkout

B actions/setup-node

C docker/build-push-action

D actions/upload-artifact

The 'docker/build-push-action' is designed to build Docker images and push them to registries.

What triggers a GitHub Actions workflow by default?

A A push to the repository

B Opening a new issue

C Deleting a branch

D Viewing the repository

Workflows often trigger automatically when you push code changes to the repository.

Why use Docker containers in CI/CD pipelines?

A To slow down the build process

B To run code in a consistent environment

C To avoid using version control

D To delete code automatically

Docker containers ensure your app runs the same way everywhere by packaging all dependencies.

Where can Docker images be pushed from GitHub Actions?

A Docker Hub or other container registries

B GitHub Issues

C GitHub Wiki

D Local text files

Docker images are pushed to container registries like Docker Hub for storage and sharing.

Explain how you would set up a GitHub Actions workflow to build and push a Docker image when code is pushed.

Think about the steps from code push to Docker image upload.

Describe the benefits of using GitHub Actions with Docker in a software project.

Consider how automation and containers improve development.

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