Your First Container

docker run hello-world

That one command did four things. Let's dissect it, because docker run is the command you'll use most, and its flags are worth understanding rather than memorizing.

What just happened

Docker looked for an image named hello-world locally. Didn't find it.
It downloaded ("pulled") it from Docker Hub, the public image registry.
It created a container from the image and started it.
The container printed its message and exited.

Check (typical output, abbreviated):

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
...
Hello from Docker!
This message shows that your installation appears to be working correctly.

The Unable to find image locally line isn't an error — it's Docker telling you step 1 failed over to step 2.

The flags that matter

docker run -it ubuntu bash

-i — keep stdin open (so you can type)
-t — give me a terminal (so it looks right)
-it together — "I want to interact with this thing"

That command downloads Ubuntu, starts a container, and drops you at a bash prompt inside it. Type exit to leave. The container stops when you exit.

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

-d — detached: run in the background, give me my terminal back
--name myweb — call it myweb instead of a random name (Docker's random names like jolly_morse are charming once)
-p 8080:80 — publish port 80 inside the container as port 8080 on your machine (more in Ports and Networking)

Check: After that command, visiting http://localhost:8080 in your browser shows the nginx welcome page.

docker run --rm hello-world

--rm — delete the container when it exits. For throwaway runs, always --rm, or stopped containers pile up (see Day-to-Day Docker).

Reading a run command left to right

docker run [options] IMAGE [command]

Everything before the image name configures how to run; the image name says what to run; anything after is the command to run inside (overriding the image's default).

Next: Images vs. Containers