Creating images

There are several reasons to make your own docker image, but mostly there are two. The application you want to run does not have a docker image available, or there is an image available, but it is not working on OpenShift. Due to the fact that OpenShift is designed to be a shared cluster, where users from different teams will run applications in the same hardware, OpenShift has to add limitations and runs things differently than in a standard Kubernetes cluster.

Building images locally

In this example we are going to use the official nginx image built over the Alpine Linux distribution, and make the necessary changes to make it work in OpenShift.

Three steps are needed to run build an image locally in a computer.

  • First a Dockerfile must be written, for example this:
FROM nginx:alpine

# support running as arbitrary user which belogs to the root group
RUN chmod g+rwx /var/cache/nginx /var/run /var/log/nginx && \
    chown nginx.root /var/cache/nginx /var/run /var/log/nginx && \
    # users are not allowed to listen on privileged ports
    sed -i.bak 's/listen\(.*\)80;/listen 8081;/' /etc/nginx/conf.d/default.conf && \
    # Make /etc/nginx/html/ available to use
    mkdir -p /etc/nginx/html/ && chmod 777 /etc/nginx/html/ && \
    # comment user directive as master process is run as user in OpenShift anyhow
    sed -i.bak 's/^user/#user/' /etc/nginx/nginx.conf

WORKDIR /usr/share/nginx/html/
EXPOSE 8081

USER nginx:root

The Dockerfile is:

  1. Giving write permissions to the root group (not the root user) to several folders that nginx needs to write to (/var/cache/nginx, /var/run, /var/log/nginx, and /etc/nginx/html/). Applications are run using a random user and the root group.
  2. Changing the port where nginx listens to, as only root is allowed to listen on privileged ports (<1024).
  3. And finally comment out the user configuration directive.

The original nginx:alpine image has 5 layers, and we will adding a new one (RUN).

A simpler example of Dockerfile could be:

FROM alpine

RUN apk add git

This is just installing git over alpine, and add also a new layer.

See the Dockerfile reference docs.

Then, the following command must be used to build the image docker.io/user/name:tag:

docker build . -t docker.io/user/name:tag

And finally, to publish the image:

docker push docker.io/user/name:tag

Using Rahti to build container images

This assumes that you have authorized a Rahti command line session and created a project in Rahti. Instructions for that are shown in Chapter Command line tool usage.

Steps:

Create Rahti specific definitions with oc new-build command. Be sure not to be in a directory under git version control:

$ oc new-build --to=my-hello-image:devel --name=my-hello --binary
    * A Docker build using binary input will be created
      * The resulting image will be pushed to image stream tag "my-hello-image:devel"
      * A binary build was created, use 'start-build --from-dir' to trigger a new build

--> Creating resources with label build=my-hello ...
    imagestream.image.openshift.io "my-hello-image" created
    buildconfig.build.openshift.io "my-hello" created
--> Success

Then you need a Dockerfile, you can use any of the previous Dockerfile in the previous example, or any other one you may have around. In order to tell OpenShift to build the image, just cd to the folder where the Dockerfile is, and start build with the oc start-build command, it will take any file in the current directory and output the build process to local terminal:

oc start-build my-hello --from-dir=./ -F

The image will appear in the Rahti registry console registry-console.rahti.csc.fi/registry, and it will be visible to internet at docker-registry.rahti.csc.fi/<project-name>/my-hello:devel for docker compatible clients.

For command line usage with docker compatible clients, the docker repository password will be the access token shown when authorizing Rahti command line session and user name can be unused.

The Docker CLI tool login instructions are also shown in the Rahti registry console.

Using the Source to Image mechanism

OpenShift allows to build and deploy code without writing a Dockerfile. This is called Source to Image or s2i. For example, use the official python sample code:

$ oc new-app https://github.com/OpenShiftDemos/os-sample-python.git
--> Found image 4e4d991 (2 weeks old) in image stream "openshift/python" under tag "3.8" for "python"

    Python 3.8 
    ---------- 
    Python 3.8 available as container is a base platform for building and running various Python 3.8 applications and frameworks. Python is an easy to learn, powerful programming language. It has efficient high-level data structures and a simple but effective approach to object-oriented programming. Python's elegant syntax and dynamic typing, together with its interpreted nature, make it an ideal language for scripting and rapid application development in many areas on most platforms.

    Tags: builder, python, python38, python-38, rh-python38

    * The source repository appears to match: python
    * A source build using source code from https://github.com/OpenShiftDemos/os-sample-python.git will be created
      * The resulting image will be pushed to image stream tag "os-sample-python:latest"
      * Use 'start-build' to trigger a new build
    * This image will be deployed in deployment config "os-sample-python"
    * Port 8080/tcp will be load balanced by service "os-sample-python"
      * Other containers can access this service through the hostname "os-sample-python"

--> Creating resources ...
    imagestream.image.openshift.io "os-sample-python" created
    buildconfig.build.openshift.io "os-sample-python" created
    deploymentconfig.apps.openshift.io "os-sample-python" created
    service "os-sample-python" created
--> Success
    Build scheduled, use 'oc logs -f bc/os-sample-python' to track its progress.
    Application is not exposed. You can expose services to the outside world by executing one or more of the commands below:
     'oc expose svc/os-sample-python' 
    Run 'oc status' to view your app.

The image will be found in Rahti's registry. Then do as suggested and expose the new application to the outside world:

$ oc expose svc/os-sample-python
route.route.openshift.io/os-sample-python exposed

In order to get the new route hostname do:

oc get route os-sample-python

If you enter the hostname in a browser, you will see the "Hello World!" message.

A new build can be triggered in the command line:

oc start-build os-sample-python

Or using webhooks

Last edited Fri Aug 14 2020