docs: use ruby docker image for building docs

Get away with the jekyll:3.8 image which is already four years old.  Use
the ruby instead. The jekyll image did not bring any value (more
problems, if anything) as we install/update jekyll and all other gems
with byndler nevertheless (image was jekyll:3.8 but we use jekyll
v3.9.3).

Makefile

@@ -16,13 +16,15 @@ BASE_IMAGE_MINIMAL ?= gcr.io/distroless/base
 # Use host networking because 'jekyll serve' is stupid enough to use the
 # same site url than the "host" it binds to. Thus, all the links will be
 # broken if we'd bind to 0.0.0.0
-JEKYLL_VERSION := 3.8
+RUBY_IMAGE_VERSION := 3.1
 JEKYLL_ENV ?= development
 SITE_BUILD_CMD := $(CONTAINER_RUN_CMD) --rm -i -u "`id -u`:`id -g`" \
+	$(shell [ -t 0 ] && echo '-t') \
 	-e JEKYLL_ENV=$(JEKYLL_ENV) \
-	--volume="$$PWD/docs:/srv/jekyll" \
+	--volume="$$PWD/docs:/work" \
 	--volume="$$PWD/docs/vendor/bundle:/usr/local/bundle" \
-	--network=host jekyll/jekyll:$(JEKYLL_VERSION)
+	-w /work \
+	--network=host ruby:$(RUBY_IMAGE_VERSION)
 SITE_BASEURL ?=
 SITE_DESTDIR ?= _site
 JEKYLL_OPTS := -d '$(SITE_DESTDIR)' $(if $(SITE_BASEURL),-b '$(SITE_BASEURL)',)