Keep Amazon kindlegen_linux_2.6_i386_v2_9 available for the docker image because Amazon writes:

KindleGen is no longer available for download.

Please use Kindle Previewer to convert, preview, and validate your eBooks. Kindle Previewer provides the same functionality of KindleGen and, in addition, provides:

Latest Kindle Conversion software that provides up-to-date validation for Enhanced Typesetting books (currently available for books in all languages except Simplified Chinese, Japanese, and Russian). Choice of using in Graphical User Interface (GUI) for a visual inspection of your book, or, Command Line Interface (CLI) mode for bulk validation. Faster preview and validation with features like thumbnails, auto-previews, and filters for pages with images, tables, drop caps, and links.

We recommend using EPUB format for publishing new reflowable titles and updating previously published titles. MOBI should only be used when testing on older devices that do not support Enhanced Typesetting.