Separate bundle into earlier step.

Copying just the bundler configurations and running bundle install in
the earlier step dramatically cuts down on iteration time since the
bundle install will only re-run if the gems actually change.

Earthfile

@@ -1,16 +1,21 @@
 VERSION 0.8
 
+bundle:
+	FROM docker.io/library/ruby:3.3
+	COPY --dir Gemfile Gemfile.lock \
+		/src/site
+	WORKDIR /src/site
+	RUN bundle install
+
 nanoc:
 	#FROM docker.io/library/ruby:3.3-alpine
-	FROM docker.io/library/ruby:3.3
+	FROM +bundle
 	COPY --dir content layouts lib Gemfile Gemfile.lock Rules nanoc.yaml \
 		/src/site/
-	WORKDIR /src/site
 
 
 build:
 	FROM +nanoc
-	RUN bundle install
 	RUN bundle exec nanoc
 	SAVE ARTIFACT output AS LOCAL output