Docker · Containers in practice
What is the difference between an image and a container?
Short answer
An image is an immutable, layered filesystem template. A container is a running instance of that image with a writable layer and its own process, network and mounts.
An image is built once and does not change. Each Dockerfile instruction produces a layer, and the image is identified by a digest derived from its content. A container adds a thin writable layer on top, so writes at runtime do not modify the image or affect other containers started from it.
docker build -t api:1.0.0 .
docker run -d --name api-1 api:1.0.0
docker ps
docker exec api-1 sh -c "echo hello > /tmp/probe"Practical consequences
- Rebuilding is how you change an image; you do not patch a running container
- The writable layer is tied to the container and disappears with it
- Data that must survive belongs in a volume, not the container layer
- Many containers can run from the same image with different configuration
Image tags are mutable labels, but digests are immutable. Deploying by digest avoids the surprise of a tag moving under you, which is why production systems often pin digests.
Follow-ups you should expect
Interviewers use these to check whether the first answer was understanding or memorisation.
- 1Where do writes made inside a running container go?
- 2Why is the writable layer a bad place for persistent data?
- 3What is the difference between an image tag and a digest?
- 4Can two containers share the same image safely?
Say it in the interview
An image is an immutable layered filesystem template, and a container is a running instance of it with an extra writable layer. Writes at runtime go to that layer, so they vanish when the container is removed and do not affect the image or other containers. That is why I keep persistent state in volumes and deploy images by immutable digest in production.
Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.
Related questions
Reading is step one
Now say it out loud.
Understanding an answer and delivering it under pressure are different skills. Rehearse this topic in a live Docker mock interview and get scored feedback on depth, clarity and confidence.