Docker from image to running container

A container image is an immutable package of filesystem layers and metadata; a container is a running instance with a writable layer, process, network, and configured resources.

At a glance

Question	Practical answer
When is it useful?	One web image can start separate development and production containers with different environment values while sharing the same application bytes.
What should you do?	Build a tiny HTTP image, inspect its layers, run it with an explicit port and read-only configuration, then stop and remove only the test container.
How do you know it worked?	The service responds on the mapped port, the image digest is known, logs appear on standard output, and a fresh container behaves the same.
Common failure	Containers are process isolation, not a complete security boundary; use minimal images, non-root users, scanning, and constrained privileges.

flowchart LR
  A[Question] --> B[Docker from image to running container]
  B --> C[Small example]
  C --> D[Evidence]

The important idea is not to stop at a definition: connect the concept to a small example and observable evidence.

Worked example

One web image can start separate development and production containers with different environment values while sharing the same application bytes.

Before acting, write the success signal. Change one condition at a time, observe the result, and record assumptions. For Docker from image to running container, this separates what you know from what you are merely guessing.

Practice in 20–30 minutes

Goal: Build a tiny HTTP image, inspect its layers, run it with an explicit port and read-only configuration, then stop and remove only the test container.

1. Record the starting state and your prediction.
2. Implement the smallest version without adding unnecessary tools.
3. Change exactly one input or constraint and repeat.
4. Save a command, screenshot, output, or checklist as evidence.

Expected result: The service responds on the mapped port, the image digest is known, logs appear on standard output, and a fresh container behaves the same.

What can go wrong

Containers are process isolation, not a complete security boundary; use minimal images, non-root users, scanning, and constrained privileges.

When the result differs from your prediction, do not change many things at once. Check inputs, versions, environment, permissions, and logs, then repeat from the smallest example.

Definition of done

• I can explain the concept in my own words.
• I completed the small example and kept evidence.
• I know one failure mode and how to check it.
• Someone else can repeat the work without guessing missing steps.

Go deeper

Use the linked resource or repository at the end of the page when you need a full implementation. Check current versions before applying commands to a real project.