I'm not sure why this fixes the image build, but I found others on the internet suggesting that using "the current" version image fixes this kind of issue and it indeed fixed the issue and I was able to build the image with this adjustment.
FabCal
Renders a web calendar, sanitizing the input and serving it from a static URL. Also renders HTML pages for embedding in homepages.
Description
Languages
Python
42.7%
CSS
30.4%
HTML
25.2%
Dockerfile
1.7%