alt attribute on img tag is missing

The `alt` attribute on `img` tags is always missing. I have expected to set the `alt` attribute by adding a caption (`figcaption`). But that's not happening. Especially for SEO optimised sites the alt attribute is very important. How can I set it? Anny help is appreciated!

Reproduce the behavior in the Trix rich text editor by adding an image with a caption, then trace how image content is represented and rendered. Confirm how captions and image attributes are currently handled. Done means an image can receive the expected alt attribute and the behavior is covered by an appropriate regression test.

Written by the indexing model from the issue text.