Understanding Images vs Containers (The Big One)¶

If you remember only one idea from this whole course, make it this one. Almost every confusing Docker moment comes from mixing up images and containers - so let's make the difference crystal clear. 💡

What we will do (in very simple steps)¶

1. Learn the difference between an image and a container
2. Prove it by running the same image twice
3. Show that removing containers doesn't remove the image

The mental model 🍱¶

Back to our kitchen:

• An image is a meal kit sitting on a shelf. It's a template - the recipe plus all the packed ingredients. It never changes, and you can keep as many copies as you like.
• A container is a meal you actually cook and serve from that kit. It's alive, it does its job, and when you're done you clear it away.

The key insight: one image can make many containers. You can cook the same meal kit again and again - each cooked meal is separate, even though they all came from the same kit.

👩‍💻 If you've programmed before: an image is like a class, and a container is like an object (an instance) created from that class. Same idea.

Let's prove it¶

We'll run the same nginx image twice, creating two separate containers.

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

Now look at your running containers:

docker ps

You'll see two containers - web1 and web2 - both made from nginx:

CONTAINER ID   IMAGE   STATUS         NAMES
f1e2d3c4b5a6   nginx   Up 3 seconds   web2
a1b2c3d4e5f6   nginx   Up 5 seconds   web1

Now look at your images:

docker images

You'll see just one nginx image:

REPOSITORY   TAG       IMAGE ID       SIZE
nginx        latest    a1b2c3d4e5f6   188MB

There it is: two containers, one image. 🎯 The single meal kit on the shelf, cooked twice.

Removing containers doesn't remove the image¶

Let's clear away both containers (the -f forces them to stop and be removed in one go):

docker rm -f web1 web2

Check they're gone:

docker ps

Empty - no running containers. But now check your images again:

docker images

The nginx image is still there. 🧊

That's the meal kit still on the shelf after you've cleared away the cooked meals. You can cook from it again any time, without re-downloading anything.

The cheat sheet¶

✅ Checkpoint¶

You've got this lesson if you can answer:

• You ran nginx twice. How many images did that use? (One.)
• How many containers did it create? (Two.)
• After docker rm -f web1 web2, does docker images still show nginx? (Yes - removing containers never removes the image.)

If those make sense, the hardest concept in Docker is now behind you. 🙌

🩹 Common hiccups¶

• "name is already in use": you already have a web1 or web2 from earlier. Remove it with docker rm -f web1 and try again.
• docker images shows extra images: that's fine - it lists every image you've ever pulled, including hello-world from lesson one.
• Mixing up the two commands: remember docker images = the shelf (templates), docker ps = what's cooking right now (running containers).

Next up: Basic Container Lifecycle - how containers are born, paused, restarted, and removed, and why the changes you make inside them can disappear.