[docs] Add missing RUN command from custom docker config
[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 #21973
strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc