Understanding Containers

What is a Container?

A container is a standard unit of software that packages code and all its dependencies so it runs consistently across computing environments.

-- Docker

Unlike virtual machines, which are slow to boot, containers start in seconds and are much more lightweight.

Virtual Machines vs Containers

Virtual machines (VMs) have been around for a long time. They run a full OS on top of your host machine using a hypervisor. Each VM has its own:

Full guest operating system
Allocated CPU, memory, and storage
Separate system updates and configs

VMs work but are resource-heavy. Containers, on the other hand, share the host OS kernel, keeping apps isolated while being lightweight.

Advantages of Containers:

Start in seconds vs minutes for VMs
Lower resource usage — multiple containers on the same host OS
Same behavior across environments — dev, test, production

Images vs Containers

A container is basically a running instance of an image:

Image: Read-only blueprint
Container: Running instance with read-write filesystem

You can launch multiple containers from a single image. Think of it like a class (image) and objects (containers).

Pull Docker Image

Download the "nginx" image:

$ docker pull library/nginx

Check the images you have:

$ docker images

You can also see the image in Docker Desktop under the "Images" tab.

Run a Container

Start a container from an image:

$ docker run -d -p hostport:containerport namespace/name:tag

Meaning:

-d: Run detached (in background)
-p: Forward host port to container port
hostport: Port on your machine
containerport: Port inside container
namespace/name:tag: Image and version

Example:

$ docker run -d -p 3000:80 library/nginx:latest

$ docker ps

Visit http://localhost:3000 to see the running app.

Stop a Container

Two main ways to stop a container:

docker stop: Sends SIGTERM, graceful shutdown
docker kill: Sends SIGKILL, forceful stop

Example:

$ docker ps

$ docker stop CONTAINER_ID

Refreshing the webpage now should show the server is down.

Running Multiple Containers

Containers are lightweight. You can run many on the same host.

Example: Start 2 instances of the "nginx" container on different ports:

$ docker run -d -p 3000:80 library/nginx

$ docker run -d -p 4000:80 library/nginx

Open each port in the browser to verify:

Use docker ps to see all running containers.

Note:

Each container is fully isolated. Even if multiple containers use the same image, changes in one container do not affect others unless you use shared volumes.

Key Takeaways

Containers package apps and dependencies in isolated, lightweight environments.
Images define containers; containers are running instances.
Containers start fast and use fewer resources than virtual machines.
You can run multiple containers from the same image simultaneously.