Simplify gemrc nodoc

Per https://stackoverflow.com/questions/1381725/how-to-make-no-ri-no-rdoc-the-default-for-gem-install#comment22636260_7662245
this is possible since rubygems 2.0.0 from 2012.

I updated the comment to indicate this is only for the `gem` command,
bundler already doesn't do this by default.

It works:
```
$ gem install rubocop -v 1.69.0
Fetching rubocop-1.69.0.gem
Successfully installed rubocop-1.69.0
1 gem installed
$ gem update rubocop
Updating installed gems
Updating rubocop
Fetching rubocop-1.69.1.gem
Successfully installed rubocop-1.69.1
Gems updated: rubocop
```

Author: Earlopain

3.1/alpine3.20/Dockerfile

@@ -41,13 +41,10 @@ RUN set -eux; \
 	rm -rf /var/lib/apt/lists/*
 
 {{ ) else "" end -}}
-# skip installing gem documentation
+# skip installing gem documentation with `gem install`/`gem update`
 RUN set -eux; \
 	mkdir -p /usr/local/etc; \
-	{ \
-		echo 'install: --no-document'; \
-		echo 'update: --no-document'; \
-	} >> /usr/local/etc/gemrc
+	echo 'gem: --no-document' >> /usr/local/etc/gemrc
 
 ENV LANG C.UTF-8