Docker Kata 6: Creating Docker Images
Learn to create an image from a modified container and with Dockerfile.
We'll cover the following...
The previous katas have used images provided on Docker Hub. Most of those images are created and managed by software vendors.
This kata will demonstrate how we can define and build our own images for our own use or to distribute to others. This kata will cover two ways to create an image:
- Commit changes to a modified container.
- Build an image from a Dockerfile.
Step 1: Create an image from a modified container
First,
mkdir dockerimagecd dockerimagecp ../index.html .ls
The output will be something like this:
Commands
Parameter/Command | Description |
| This creates, and then changes to, a new directory called |
| This copies the |
| This lists the files in the |
These commands create a new directory called dockerimage and copy a sample index.html file into that directory.
The command to run a disconnected NGINX container is given below.
docker container run --name web -d -p 80:80 nginx
If we run the command above, the output will be something like this:
Commands
Parameter | Description |
| This is the parent command. |
| This runs a container. |
| This assigns a name to a container. |
| This is the name to assign to the container. |
| This runs a container in disconnected mode. |
| This publishes a port from the container to the host. |
| This publishes port 80 on the container to port 80 on the host. |
| This is the name of the image to run. |
This command starts an NGNIX container using the same command from Kata 4.
The command to copy a file into the container is given below.
docker container cp index.html web:/usr/share/nginx/html/index.html
After execution of the command above, the result will be something like this:
Commands
Parameter | Description |
| This is the parent command. |
| The |
| This is the name of the file to copy into the container. |
| This is the image and the path. The name of the container to which the file should be copied is before the colon. The path inside the container to which the file should be copied follows the colon. The effect is that the |
The cp command copies files between containers and their hosts. This step copied the index.html file from the host to the container.
The command to open the NGINX server page is given below.
curl localhost
The result will be something like this:
The default NGINX welcome HTML we’ve seen previously has been replaced with the DevOps Katas custom page.
The curl command issues a request to the local system via HTTP. We can also use Firefox to view this page. Note that the welcome page has been modified. The index.html file copied in the previous step overwrote the default NGINX welcome page.
The command to list all the running containers is provided below.
docker container ls
The result of the command above will be something like this:
Commands
Parameter | Description |
| This is the parent command. |
| This lists the running containers when combined with the |
This step lists all the running containers. The NGINX container we ran in an earlier step is running.
The command to create an image from the container is given below.
docker container commit web kataimage_nginx
The output will be something like this:
Commands
Parameter | Description |
| This is the parent command. |
| The |
| This is the name of the container from which to derive a new image. |
| This is the name assigned to the new image. |
The commit command is used to commit the changes to a container, thus creating a new image.
Recall in Kata 3 when we learned about file system changes in a container. When we copied the index.html file from the host to the NGINX container, we modified the container, but not the image from which that container was started. The commit command creates a new image called kataimage_nginx that includes that change.
The command to list all the images is given below.