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Create Custom Images

In this lab we are going to see how an image is built:

  • An image is specified on a file called Containerfile or Dockerfile
  • An image bases on a base image
  • A base image provides a common basis for building an image
  • Each image is composed of layers
  • Each instruction on the Containerfile adds a layer

This is an example of a Containerfile:

FROM redhat/ubi8
# The FROM command tell us which is the base image. We are using a dockerhub image published by RedHat. 
# https://hub.docker.com/r/redhat/ubi8

LABEL author="Eva Ramon [email protected]"
LABEL description="My very own Apache Server"
# The LABEL command is useful to provide information about the image

RUN yum install -y httpd &&\
    yum clean all
# The RUN command is executed during build
# Each RUN command adds a layer to the image, so it's a good practice to use && to combine commands 

EXPOSE 80
# The EXPOSE command says which port on the container should be exposed (metadata only)

ENTRYPOINT ["httpd"]
# The ENTRYPOINT is the command to be executed on the container when it is started 
# The entrypoint can't be overriden

CMD ["-D", "FOREGROUND"]
# The CMD command provides the parameters for the command specified in ENTRYPOINT
# The arguments can be overriden by passing them at the end of podman run

Copy the content and save it as "Containerfile" or as "Dockerfile".

Alternatively, you can download it directly from here:

wget https://raw.githubusercontent.com/IBM/csm-dach-cloud-native-labs/main/day-1/Containerfile

NOTE: Notice that we talk from a Dockerfile or Containerfile arbitrarly. Both names are interchangeable and accepted by podman.

Explanations to the different commands are included in the file as comments.

Our Containerfile is ready - Let's build the image with podman build:

user1$ podman build -t myapache:0.1 .

STEP 1: FROM redhat/ubi8-minimal
✔ docker.io/redhat/ubi8-minimal:latest
Trying to pull docker.io/redhat/ubi8-minimal:latest...
...
Successfully tagged localhost/myapache:0.1
8997b9661025a5baa1b0fc5c4dfc73508ff32eb26afaa34ce752098c6f7fd58b

Please don't miss the dot (.) since it indicates the location of the source, in this case the current directory. The Containerfile must be in the same directory where the build command is ran.

In the list of images, we see both the image we created and the base image:

user1:~$ podman images

REPOSITORY             TAG         IMAGE ID      CREATED        SIZE
localhost/myapache     0.1         8997b9661025  2 minutes ago  292 MB
docker.io/redhat/ubi8  latest      cc0656847854  3 weeks ago    235 MB

Run a container from this image:

user1:~$ podman run --name myapache -d -p 8081:80 myapache:0.1

278e7d99275e981315f06c5517847d810e80b6153eaed30a29134106771129b7
user1:cloud-native-labs$ podman ps 
CONTAINER ID  IMAGE                   COMMAND        CREATED        STATUS            PORTS                 NAMES
278e7d99275e  localhost/myapache:0.1  -D FOREGROUND  3 seconds ago  Up 3 seconds ago  0.0.0.0:8081->80/tcp  myapache

NOTE: in order to avoid port conflicts, please use as the local port (the one on the left of the ':') your user number + 8000. For example, user 1 will use 8001.

We created a new container based on our custom image, with name myapache, in dettached mode and forwarding port 8080 on the host to port 80 on the container.

Should we try it?

user1:cloud-native-labs$ curl localhost:8081

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
...
		<title>Test Page for the HTTP Server on Red Hat Enterprise Linux</title>
...

It looks like it's working :)

We saw in the Containerfile that there is an ENTRYPOINT and a CMD commands. Both commands are similar but there is an important difference:

  • Both ENTRYPOINT and CMD are executed when the container is created
  • ENTRYPOINT is the command and CMD are the parameters, appended at the end
  • You can override the CMD with podman run passing the parameters when running podman run
  • You can't override the entrypoint: instead anything added to the end of podman run is appended to the command

Let's try overriding CMD:

user1:~$ podman run --name override myapache:0.1 -v

Server version: Apache/2.4.37 (Red Hat Enterprise Linux)
Server built:   Oct 26 2021 14:18:06

What did we do? Instead starting the web server by executing httpd -d foreground we printed out the apache version by calling httpd -v. The version was printed and the container finished, so the container is not running anymore:

user1:~$ podman ps -a

CONTAINER ID  IMAGE                   COMMAND        CREATED         STATUS                    PORTS                 NAMES
...
e42a2930ed34  localhost/myapache:0.1  -v             2 minutes ago   Exited (0) 2 minutes ago  0.0.0.0:8081->80/tcp  override

To finish a little housekeeping:

user1:~$ podman stop --all

ac05d949910abc9ffe04ed8d34d2a4c2024de13ae234dc16233a92b2c3c9250f

user1:~$ cloud-native-labs$ podman rm --all

ac05d949910abc9ffe04ed8d34d2a4c2024de13ae234dc16233a92b2c3c9250f

user1:~$ podman rmi --all

Untagged: docker.io/redhat/ubi8:latest
Untagged: localhost/myapache:0.1
Deleted: ec600407b590f735212f408bf86a7cfd460e6480f0e8319b19620f1b3c8349cd