[docs] Add missing RUN command from custom docker config
Fix the `Dockerfile` example in the `Customizing image` third configuration
method by adding the missing RUN instruction.
Originally reported by Shankar Vasudevan (@vshank77).
Relates
@@ -248,7 +248,7 @@ In some environments, it may make more sense to prepare a custom image containin
FROM docker.elastic.co/elasticsearch/elasticsearch:{version}
ADD elasticsearch.yml /usr/share/elasticsearch/config/
USER root
-chown elasticsearch:elasticsearch config/elasticsearch.yml
+RUN chown elasticsearch:elasticsearch config/elasticsearch.yml
USER elasticsearch
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