Use ruby:3 as image to serve dev

github-pages isn't compatible with Ruby 4, since its dependency
jekyll-commonmark depends on a very old version of commonmarker, which isn't
compatible with Ruby 4.

The latest release of jekyll-commonmark was in January 2022, so maybe this is
a sign that we'll have to look for alternatives.

Signed-off-by: Pablo Zmdl <pablo@nextcloud.com>

Author: pabzm

Makefile

@@ -9,4 +9,4 @@ css:
 	npx less --clean-css="--s1 --advanced"  styles/styles.less > styles/styles.min.css
 
 serve-dev:
-	docker run -it -p 4000:4000 -v $(PWD):/site -w /site library/ruby bash -c 'bundle install && bundle exec jekyll serve -H 0.0.0.0'
+	docker run -it -p 4000:4000 -v $(PWD):/site -w /site library/ruby:3 bash -c 'bundle install && bundle exec jekyll serve -H 0.0.0.0'