Posting in case this helps anyone in the future. Not particularly difficult, I was just having this issue recently, and took some time today to work around it.

Lately, my Calibre-Web-Automated container will update eBook metadata, but will not update the book cover. I was never able to find an error, and using the CLI cover-enforce command didn’t do anything either. I’m pretty sure that’s all the GUI is doing, so not a surprise the CLI didn’t work. Instead, it will insert a generic cover. For some lesser-known books, where it cannot find an appropriate cover, this procedure will also help.

First, ID the cover you want. If you have to manually scan the cover and get it into your container, you’ll need to sort that out yourself. I was able to find decent enough covers on DDG image search. From there, drop into a shell in the CWA container, and run the following commands (Substitute variables below to meet your needs):

cd /calibre-library/${AUTHOR}/${BOOK_TITLE}  
curl "${IMAGE_URL}" > cover.jpg  
ebook-meta "${EBOOK_FILE}" --cover cover.jpg  

This assumes the image is a JPEG. Sub as necessary. Some that I found were WEBP files, and I wasn’t sure they would work properly with ebook readers, so I just stuck with good old JPEG. Also, I figured the naming and placement of the cover file would scale fine for my whole library where needed. Adjust as you see fit.

ETA: Corrected a variable