Running tests in containers in Docker - Step-by-Step Execution

Process Flow - Running tests in containers

Write test code

↓

Create Dockerfile with test environment

↓

Build Docker image

↓

Run container from image

↓

Execute tests inside container

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View test results

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Stop and remove container

This flow shows how to write tests, build a Docker image with the test setup, run tests inside a container, and then view results.

Execution Sample

Docker

FROM python:3.12-slim
WORKDIR /app
COPY . /app
RUN pip install pytest
CMD ["pytest"]

Dockerfile sets up a Python 3.12 environment, copies test code, installs pytest, and runs tests when container starts.

Process Table

Step	Action	Command/Operation	Result/Output
1	Write test code	Create test_example.py with simple test	Test file ready
2	Create Dockerfile	Write Dockerfile with Python and pytest	Dockerfile ready
3	Build image	docker build -t test-image .	Image 'test-image' created
4	Run container	docker run --rm test-image	Container starts and runs pytest
5	Execute tests	pytest runs inside container	Tests pass or fail output shown
6	View results	Output displayed in terminal	Test summary visible
7	Container stops	Container exits after tests	Container removed (--rm flag)

💡 Container stops after tests complete and --rm flag removes it automatically

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 7
test code	none	test_example.py created	copied into image	used inside container	gone with container
Docker image	none	none	test-image built	used to run container	exists locally
container	none	none	none	running tests	stopped and removed

Key Moments - 3 Insights

Why do we use the --rm flag when running the container?

The --rm flag automatically removes the container after tests finish, keeping your system clean as shown in step 7 of the execution table.

How does the test code get inside the container?

The test code is copied into the image during build (step 3), so when the container runs (step 4), it has the test files ready to execute.

What happens if tests fail inside the container?

The pytest output will show failure details in step 5, but the container still stops and is removed as per step 7.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the Docker image created?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Check the 'Build image' action in the execution table rows

According to the variable tracker, what happens to the container after step 7?

A It is stopped and removed

B It is stopped but remains on disk

C It keeps running

D It restarts automatically

💡 Hint

Look at the 'container' variable state after step 7 in the variable tracker

If you remove the COPY command from the Dockerfile, what changes in the execution?

A Tests will run successfully

B Tests will fail because test code is missing

C Image build will fail

D Container will not start

💡 Hint

Refer to how test code is copied into the image in the execution table and variable tracker

Concept Snapshot

Running tests in containers:
- Write test code and Dockerfile
- Build image with test environment
- Run container to execute tests
- Use --rm to auto-remove container
- View test results in terminal output

Full Transcript

This visual execution shows how to run tests inside Docker containers. First, you write your test code and create a Dockerfile that sets up the test environment. Then you build a Docker image that includes your tests. Running a container from this image executes the tests inside it. The container outputs test results to your terminal and stops automatically if you use the --rm flag, which also removes the container to keep your system clean. The test code is copied into the image during build, so it is available inside the container. Even if tests fail, the container stops and is removed as expected.