Docker execution in Cypress - Build an Automation Script

Run Cypress tests inside Docker container

Preconditions (3)

Step 1: Build the Docker image using the Dockerfile

Step 2: Run the Docker container from the built image

Step 3: Inside the container, execute Cypress tests

Step 4: Observe the test execution output in the container logs

✅ Expected Result: Cypress tests run successfully inside the Docker container and test results are visible in the container logs

Automation Requirements - Cypress

Assertions Needed:

Verify Cypress tests pass inside Docker container

Verify container exits with code 0 indicating success

Best Practices:

Use official Cypress Docker image as base

Mount project folder as volume for test code

Use explicit commands to run Cypress tests

Capture and assert exit code of container

Automated Solution

Cypress

const { execSync } = require('child_process');

describe('Docker execution of Cypress tests', () => {
  it('should build Docker image and run Cypress tests successfully', () => {
    // Build Docker image
    execSync('docker build -t cypress-test-image .', { stdio: 'inherit' });

    // Run Docker container and capture output
    const output = execSync('docker run --rm cypress-test-image', { encoding: 'utf-8' });

    // Check output contains test pass indication
    expect(output).to.include('All specs passed!');
  });
});

This Cypress test script uses Node.js execSync to run Docker commands synchronously.

First, it builds the Docker image named cypress-test-image using the Dockerfile in the current folder.

Then, it runs the container from that image and captures the output logs.

The test asserts that the output contains the text All specs passed! which Cypress prints when tests pass.

This confirms that Cypress tests ran successfully inside the Docker container.

Using execSync with stdio: 'inherit' streams Docker build logs to console for visibility.

Common Mistakes - 4 Pitfalls

Not using the official Cypress Docker image as base

{'mistake': 'Not mounting the project folder as a volume', 'why_bad': "Test code changes won't be reflected inside the container, causing stale tests to run.", 'correct_approach': 'Mount the project folder as a volume to ensure latest test code is used.'}

Ignoring container exit codes

Using asynchronous Docker commands without waiting

Bonus Challenge

Now add data-driven testing by running Cypress tests inside Docker with three different environment variables

Show Hint

Practice

1. What is the main benefit of running Cypress tests inside a Docker container?

easy

A. It provides a clean and consistent environment for tests.

B. It makes tests run faster on all machines.

C. It automatically fixes test failures.

D. It removes the need for writing test code.

Solution

1. Step 1: Understand Docker's role in testing

   Docker creates isolated environments that are the same everywhere.

2. Step 2: Connect Docker benefits to Cypress tests

   Running Cypress in Docker ensures tests run in a clean, consistent setup, avoiding environment differences.

3. Final Answer:

   It provides a clean and consistent environment for tests. -> Option A

4. Quick Check:

   Docker = consistent test environment [OK]

Hint: Docker ensures same environment everywhere for tests [OK]

Common Mistakes:

• Thinking Docker speeds up tests automatically
• Believing Docker fixes test code errors
• Assuming Docker removes need for tests

2. Which Docker command correctly runs Cypress tests mounting the current folder as a volume?

easy

A. docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0

B. docker run -p 8080:80 cypress/included:12.0.0

C. docker build -t cypress-test .

D. docker run cypress/included:12.0.0 --no-volume

Solution

1. Step 1: Identify volume mounting syntax

   The -v option mounts a folder; $(pwd):/e2e mounts current folder to /e2e in container.

2. Step 2: Check command correctness

   docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0 uses -v and -w to set working directory, which is correct for running Cypress tests.

3. Final Answer:

   docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0 -> Option A

4. Quick Check:

   Volume mount = -v [OK]

Hint: Use -v to mount volumes in Docker run command [OK]

Common Mistakes:

• Using -p instead of -v for volume mounting
• Forgetting to set working directory with -w
• Confusing docker build with docker run

3. Given this Docker command:

docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0

What will happen if your current folder has no Cypress tests?

medium

A. Docker will throw a syntax error and stop.

B. The container will fail to start due to missing tests.

C. Cypress will automatically create sample tests.

D. The container will run but no tests will execute.

Solution

1. Step 1: Understand volume mounting effect

   The current folder is mounted inside the container at /e2e, so container sees the files you have.

2. Step 2: Consider what happens with no tests

   If no test files exist, Cypress runs but finds nothing to execute, so no tests run but container runs fine.

3. Final Answer:

   The container will run but no tests will execute. -> Option D

4. Quick Check:

   No tests = no execution, container still runs [OK]

Hint: No tests means no runs, but container still starts [OK]

Common Mistakes:

• Expecting Docker syntax error for missing files
• Thinking Cypress auto-generates tests
• Assuming container fails to start without tests

4. You run Cypress tests in Docker but see this error:
ERROR: Cannot find module 'cypress'
What is the most likely cause?

medium

A. You forgot to mount your test folder with node_modules.

B. You used the wrong Docker image without Cypress installed.

C. Your Dockerfile syntax is incorrect.

D. Your tests have syntax errors.

Solution

1. Step 1: Analyze the error message

   The error says Cypress module is missing, meaning Cypress is not installed in the container.

2. Step 2: Check Docker image used

   If you use a Docker image without Cypress pre-installed, this error occurs. Using cypress/included images avoids this.

3. Final Answer:

   You used the wrong Docker image without Cypress installed. -> Option B

4. Quick Check:

   Missing module = wrong image [OK]

Hint: Use official cypress/included image to avoid missing Cypress [OK]

Common Mistakes:

• Blaming test syntax errors for missing module
• Assuming volume mount fixes missing Cypress
• Confusing Dockerfile syntax with missing module

5. You want to run Cypress tests in Docker and save screenshots and videos outside the container. Which command correctly achieves this?

hard

A. docker run -p 8080:80 -v $(pwd):/e2e cypress/included:12.0.0

B. docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0 --save-artifacts

C. docker run -v $(pwd):/e2e -v $(pwd)/cypress/screenshots:/e2e/cypress/screenshots -v $(pwd)/cypress/videos:/e2e/cypress/videos -w /e2e cypress/included:12.0.0

D. docker run --rm cypress/included:12.0.0

Solution

1. Step 1: Understand volume mounting for artifacts

   To save screenshots and videos outside Docker, mount their folders as volumes from host to container.

2. Step 2: Check command for multiple volume mounts

   docker run -v $(pwd):/e2e -v $(pwd)/cypress/screenshots:/e2e/cypress/screenshots -v $(pwd)/cypress/videos:/e2e/cypress/videos -w /e2e cypress/included:12.0.0 mounts the whole project plus screenshots and videos folders separately, ensuring artifacts persist on host.

3. Final Answer:

   docker run -v $(pwd):/e2e -v $(pwd)/cypress/screenshots:/e2e/cypress/screenshots -v $(pwd)/cypress/videos:/e2e/cypress/videos -w /e2e cypress/included:12.0.0 -> Option C

4. Quick Check:

   Mount artifact folders to save outside container [OK]

Hint: Mount screenshots and videos folders as volumes [OK]

Common Mistakes:

• Not mounting screenshots/videos folders separately
• Using -p port mapping instead of volume mounts
• Assuming --save-artifacts flag exists