[Docs] Use reference to docker image (#114152)

* Use reference to docker image

* Incorporate review by @jbudz

Co-authored-by: Jonathan Budzenski <jon@budzenski.me>

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Jonathan Budzenski <jon@budzenski.me>

docs/setup/docker.asciidoc

@@ -97,10 +97,10 @@ services:
 
 By default, {kib] auto-generates a keystore file for secure settings at startup. To persist your {kibana-ref}/secure-settings.html[secure settings], use the `kibana-keystore` utility to bind-mount the parent directory of the keystore to the container. For example:
 
-[source,sh]
+["source","sh",subs="attributes"]
 ----
-docker run -it --rm -v full_path_to/config:/usr/share/kibana/config -v full_path_to/data:/usr/share/kibana/data docker.elastic.co/kibana/kibana:7.14.0 bin/kibana-keystore create
-docker run -it --rm -v full_path_to/config:/usr/share/kibana/config -v full_path_to/data:/usr/share/kibana/data docker.elastic.co/kibana/kibana:7.14.0 bin/kibana-keystore add test_keystore_setting
+docker run -it --rm -v full_path_to/config:/usr/share/kibana/config -v full_path_to/data:/usr/share/kibana/data {docker-image} bin/kibana-keystore create
+docker run -it --rm -v full_path_to/config:/usr/share/kibana/config -v full_path_to/data:/usr/share/kibana/data {docker-image} bin/kibana-keystore add test_keystore_setting
 ----
 
 [float]