Custom image builds with Buildah | Builds | OpenShift Container Platform 4.2

Creating custom build artifacts
Build custom builder image
Use custom builder image

With OpenShift Container Platform 4.2, a Docker socket will not be present on the host nodes. This means the mount docker socket option of a custom build is not guaranteed to provide an accessible Docker socket for use within a custom build image.

If you require this capability in order to build and push images, add the Buildah tool your custom build image and use it to build and push the image within your custom build logic. The following is an example of how to run custom builds with Buildah.

Using the custom build strategy requires permissions that normal users do not have by default because it allows the user to execute arbitrary code inside a privileged container running on the cluster. This level of access can be used to compromise the cluster and therefore should be granted only to users who are trusted with administrative privileges on the cluster.

Prerequisites

Review how to grant custom build permissions.

Creating custom build artifacts

You must create the image you want to use as your custom build image.

Procedure

Starting with an empty directory, create a file named Dockerfile with the following content:

FROM docker.io/centos:7
RUN yum install -y buildah
# For simplicity, /tmp/build contains the inputs we’ll be building when we
# run this custom builder image. Normally the custom builder image would
# fetch this content from some location at build time. (e.g. via git clone).
ADD Dockerfile.sample /tmp/input/Dockerfile
ADD build.sh /usr/bin
RUN chmod a+x /usr/bin/build.sh
# /usr/bin/build.sh contains the actual custom build logic that will be executed when
# this custom builder image is executed.
ENTRYPOINT ["/usr/bin/build.sh"]

In the same directory, create a file named Dockerfile.sample. This file will be included in the custom build image and defines the image that will be produced by the custom build:
```
FROM docker.io/centos:7
RUN touch /tmp/built
```

In the same directory, create a file named build.sh. This file contains the logic that will be executed when the custom build runs:

#!/bin/sh
# Note that in this case the build inputs are part of the custom builder image, but normally this
# would be retrieved from an external source.
cd /tmp/input
# OUTPUT_REGISTRY and OUTPUT_IMAGE are env variables provided by the custom
# build framework
TAG="${OUTPUT_REGISTRY}/${OUTPUT_IMAGE}"


# performs the build of the new image defined by Dockerfile.sample
buildah --storage-driver vfs bud --isolation chroot -t ${TAG} .


# buildah requires a slight modification to the push secret provided by the service
# account in order to use it for pushing the image
cp /var/run/secrets/openshift.io/push/.dockercfg /tmp
(echo "{ \"auths\": " ; cat /var/run/secrets/openshift.io/push/.dockercfg ; echo "}") > /tmp/.dockercfg


# push the new image to the target for the build
buildah --storage-driver vfs push --tls-verify=false --authfile /tmp/.dockercfg ${TAG}

Build custom builder image

You can use OpenShift Container Platform to build and push custom builder images to use in a custom strategy.

Prerequisites

Define all the inputs that will go into creating your new custom builder image.

Procedure

Define a BuildConfig that will build your custom builder image:

$ oc new-build --binary --strategy=docker --name custom-builder-image

From the directory in which you created your custom build image, run the build:
```
$ oc start-build custom-builder-image --from-dir . -F
```
After the build completes, your new custom builder image is available in your project in an imagestreamtag that is named custom-builder-image:latest.

Use custom builder image

You can define a BuildConfig that uses the custom strategy in conjunction with your custom builder image to execute your custom build logic.

Prerequisites

Define all the required inputs for new custom builder image.
Build your custom builder image.

Procedure

Create a file named buildconfig.yaml. This file defines the BuildConfig that is created in your project and executed:

kind: BuildConfig
apiVersion: v1
metadata:
  name: sample-custom-build
  labels:
    name: sample-custom-build
  annotations:
    template.alpha.openshift.io/wait-for-ready: 'true'
spec:
  strategy:
    type: Custom
    customStrategy:
      forcePull: true
      from:
        kind: ImageStreamTag
        name: custom-builder-image:latest
        namespace: <yourproject> (1)
  output:
    to:
      kind: ImageStreamTag
      name: sample-custom:latest

1	Specify your project name.

Create the BuildConfig:
```
$ oc create -f buildconfig.yaml
```
Create a file named imagestream.yaml. This file defines the imagestream to which the build will push the image:
```
kind: ImageStream
apiVersion: v1
metadata:
  name: sample-custom
spec: {}
```
Create the imagestream:
```
$ oc create -f imagestream.yaml
```
Run your custom build:
```
$ oc start-build sample-custom-build -F
```
When the build runs, it launches a pod running the custom builder image that was built earlier. The pod runs the build.sh logic that is defined as the entrypoint for the custom builder image. The build.sh logic invokes Buildah to build the Dockerfile.sample that was embedded in the custom builder image, and then uses Buildah to push the new image to the sample-custom imagestream.