Building and running your image

Build the image with docker build

With the Dockerfile and hello.sh from the previous lesson in your current folder, run:

docker build -t my-hello .

Two parts to notice:

• -t my-hello gives your image a name (tag) so you can refer to it later. You can read -t as "tag it as".
• The . at the end is the build context: the folder Docker looks in for your files (the current folder here). Docker needs this to find hello.sh.

Docker runs each instruction in your Dockerfile and prints its progress. When it finishes, you have a new image called my-hello. Confirm it:

docker images

You should see my-hello in the list.

Run a container from your image

Now start a container from it, exactly like you'd run any other image:

docker run my-hello

You'll see:

Hello from inside my custom image!

That message came from your script, running inside a container built from your image. Congratulations - you've built and run your own Docker image.

Rebuilding after a change

Edit hello.sh to print something different, then build again:

docker build -t my-hello .
docker run my-hello

Because images are just built from the Dockerfile and your files, rebuilding picks up your changes. You'll notice the second build is faster - the next lesson explains why, when we look at the instructions in more detail.

Don't forget the dot

The most common docker build mistake is leaving off the . at the end. That dot is the build context - the folder Docker reads your files from - and it's required. If you run docker build -t my-hello with no path, Docker errors out asking for the context. The command is docker build -t my-hello ., dot included.

FAQ

What does the dot mean in docker build?

It's the build context: the folder Docker looks in for the files your Dockerfile copies. . means the current folder. It's required, which is why leaving it off causes an error.

What does the -t flag do?

It tags (names) the image, so you can run it by name later. docker build -t my-hello . produces an image called my-hello, which you then run with docker run my-hello.

Do I need to rebuild after changing my code?

Yes. An image is a snapshot taken at build time, so code changes only appear after you run docker build again. The rebuild is usually fast thanks to Docker's cache (next lesson).