Update documentation to Clarify Reserved Build Arguments for Image Builder

[dekiel]

Description

There has been an issue reported regarding the use of --build-arg in the image builder GitHub Actions workflow. The argument default_tag is being used, but it is overriden by the default tag generated for images during the build. The default tag is added to the build args with name "default_tag". This results in conflicts when trying to pass a custom argument with the same name.

Problem

• When using the argument --build-arg=default_tag=, it works locally but does not reflect the expected behavior in the CI pipeline, as it is overwritten by the default tag in the image building process.
• The issue occurs because default_tag is a reserved argument used for passing the default tag in to the build process in Kaniko.

Suggested Solution

• It is recommended to use a different name for custom build arguments, such as --build-arg=my_custom_tag=, to avoid conflicts with reserved arguments like default_tag.

Acceptance Criteria

• Add a note in the image builder documentation to highlight that default_tag is a reserved build argument for the default image tag and to suggest using a different name for custom arguments.
• Add information how to enable passing tags to the build args. Mention default_tag is passed always.
• Add note in image-builder github action about reserved names and values for image tags and build arguments. Llink users to the image nuilder documentation for more details.