Building Base Images

April 27, 2022 ยท View on GitHub

A base image is simply an image that has no parent. Atomic reactor can be used to build base images by using Koji's image build functionality via the add_filesystem plugin.

Add Filesystem Plugin

  1. Inspects the FROM instruction in Dockerfile. If value is "koji/image-build", this plugin proceeds. Otherwise, no further processing is performed and the plugin quits.

  2. Parses the image build configuration from image-build.conf which is used to create an image build task in Koji. A different name for the image build configuration file can be specified by using a tag in the FROM instruction. For example, FROM koji/image-build:custom.conf will load configuration from custom.conf file.`

  3. Downloads produced filesystem tarball from Koji and uses docker's import to load it as an image. The FROM instruction in Dockerfile is modified to use the ID of the newly imported image.

  4. Build proceeds as usual.

Use the from_task_id plugin parameter to use the tarball from an existing Koji task.

Image Build Configuration

The image build configuration file is used to specify how the filesystem will be created (more info is available in the koji docs). The syntax for this file is equivalent to the syntax used when in the config file used for Koji's CLI client image build subcommand: koji image-build --config config

For convenience, defaults are provided to minimize the size of the configuration file. See the DEFAULT_IMAGE_BUILD_CONF variable in add_filesystem plugin's source for all defaults. Most notably,

  • The ksurl item is by default dynamically mapped to the current branch and commit
  • The target item is by default set to the koji target parameter and should be omitted from the config file. If explicitly set in the file, the default value is overridden.

It's important to note that currently in the [factory-parameters] section, the create_docker_metadata parameter must be set to False. Setting this parameter to True, will alter the format of the tarball produced by Koji which the add_filesystem plugin does not support.