Stop 11 · CI/CD & DevOps for QATopics · 5/8

Containerized Testing (Docker Basics)

In short

'Works on my machine' is the most expensive sentence in testing. Docker packs your tests with the exact browsers and libraries they need, so they behave the same everywhere.

Running tests inside Docker containers ensures a consistent, isolated environment every time — eliminating 'works on my machine' inconsistencies between local and CI test runs.

The purple blob packs rice, dal and sabzi into a stacked steel tiffin carrier.

Think of it like this

The sealed steel tiffin

Pack rice, dal and sabzi in a steel tiffin at home and it opens the same at college, office or on a train. A Docker container packs your tests, browsers and tools together, so the CI machine gets the same lunch you packed.

Watch it happen

Life of a test container

Image

Container

Mount code

Run tests

Throw away

1/5An image is a frozen recipe: OS, Node, browsers, fonts. Playwright publishes ready-made images for this.

The big ideas

What you need to know

Image vs container

The image is the recipe, the container is one plate cooked from it. Many containers can come from one image.

Same everywhere

Laptop, teammate's laptop and CI run the same image, so browser and font differences stop causing surprise failures.

Disposable by design

Containers start fresh and get thrown away. No leftover cookies, files or cache from the last run.

Pin the version

Use an exact image tag that matches your Playwright package version, not 'latest', so updates never surprise you.

See it in code

Walk through the code

bash

1# Run your Playwright tests inside Microsoft's ready-made image2docker run --rm -it \3  -v "$(pwd)":/work -w /work \4  mcr.microsoft.com/playwright:v1.48.0-noble \5  /bin/bash -c "npm ci && npx playwright test"

1/4--rm deletes the container when it finishes. -it shows the output live.

Change v1.48.0 to match the @playwright/test version in your package.json. A mismatch means the browsers inside won't match your library.

Save yourself the pain

Common mistakes

Oops

Using the latest image tag and wondering why tests broke overnight.

Do this instead

Pin an exact tag and upgrade on purpose, together with your package version.

Oops

Running npm install on your Mac, then mounting node_modules into a Linux container.

Do this instead

Run npm ci inside the container so packages are built for the container's OS.

Your mission

Run your suite in a box

~30 min

Quick check

Why do teams run UI tests inside a Docker container in CI?

Go deeper

1Write a basic Dockerfile that packages your test suite and its dependencies.
2Run your test suite inside a container and compare consistency to running it locally.
3Learn why containerized test environments reduce a common source of flaky, environment-specific failures.