Images and containers

Learn the relationship between images and containers.

Image and container

An image is the read-only template of an application: the file system, libraries and start command. A container is a running instance of that image. Think of a class and its objects: from a single nginx image you can start as many containers as you want.

docker run -d --name web1 nginx
docker run -d --name web2 nginx    # same image, two separate containers

The writable layer

When a container starts, a thin writable layer is added on top of the image. Files created inside the container are written to this layer and disappear when the container is removed; the image never changes. Volumes are used for persistent data (you'll see them later).

docker run --name deneme alpine touch /tmp/not.txt
docker rm deneme          # /tmp/not.txt is gone too
docker run alpine ls /tmp # the image is still clean: empty

Exercise Two containers from one image

Write two commands, one per line, starting two containers named web1 and web2 from the nginx image in the background (-d).

Show example solution

docker run -d --name web1 nginx
docker run -d --name web2 nginx

Test yourself

How many containers can run from one image at the same time?
- Only one
- At most two
- None; it must be copied first
- As many as wanted
Show answer

As many as wanted — The image is read-only; each container adds its own writable layer on top.

Give the container a name.

docker run -d ____ web1 nginx

Show answer

docker run -d --name web1 nginx

Match.

image

Read-only template
container

Running instance
writable layer

Lost when the container is removed
volume

Persistent data

Next lesson: Docker architecture

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