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Docker Containers

What is a Container?

A Docker container is a running instance of a Docker image.

  • Image = Blueprint
  • Container = Running application

Container Lifecycle (With Example)

Let’s practice using the nginx image.

Step 1: Create a Container (Without Starting)

docker create --name mynginx nginx

Check:

docker ps -a

Status → Created

Step 2: Start the Container

docker start mynginx

Check:

docker ps

Status → Up (Running)

Step 3: Pause the Container

docker pause mynginx

Status → Paused

Step 4: Unpause the Container

docker unpause mynginx

Step 5: Stop the Container

docker stop mynginx

Status → Exited

Step 6: Restart the Container

docker restart mynginx

Step 7: Kill the Container

docker kill mynginx

Forcefully stops the container.

Step 8: Remove the Container

docker rm mynginx

Container is completely deleted.

Working with Running Containers

Run Nginx in Detached Mode

-d → Run in background
-p → Map port

docker run -d -p 8080:80 --name webserver nginx

Understanding Port Mapping (-p)

Format:

-p HOST_PORT:CONTAINER_PORT

Example:

-p 8080:80

  • 8080 → Host Port (your computer)
  • 80 → Container Port (inside container)

What happens?

Browser → http://localhost:8080
⬇
Host Port 8080
⬇
Container Port 80
⬇
Nginx running inside container

So:

Left side = Host
Right side = Container

Passing Environment Variables

Environment variables are used to pass configuration values into a container.

Use -e to pass environment variables.

Example 1: Passing Single Variable

docker run -e APP_ENV=production nginx

This sets:

APP_ENV=production inside the container.

Example 2: Multiple Environment Variables

docker run -e USERNAME=admin -e PASSWORD=1234 nginx

Example 3: Check Environment Variables Inside Container

Run container:

docker run -d --name envtest -e MY_NAME=Docker nginx

Enter container:

docker exec -it envtest /bin/sh

Check variable:

echo $MY_NAME

Output: Docker

Environment variables are commonly used for:

  • Database credentials
  • API keys
  • Application configuration
  • Environment settings (dev, test, prod)

View Logs

docker logs webserver

Shows output logs of the container.

View Real-Time Logs

docker logs -f webserver

Shows live logs (Press Ctrl + C to exit).

Exec into the Container

docker exec -it webserver /bin/sh

Now you are inside the container.

Try:

ls / cd /usr/share/nginx/html ls

Type exit to leave.

Run a Single Command Inside Container

docker exec webserver ls /

Runs one command without entering.

Inspect the Container

docker inspect webserver

Shows detailed information like:

  • Container ID
  • IP address
  • Port mappings
  • Environment variables