[Buildbot] #2989: Improve a bit documentation for Docker

[Buildbot trac]

#2989: Improve a bit documentation for Docker
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Reporter:  sa2ajj       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  0.9.0
 Version:  master       |   Keywords:  docs, docker
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 Per
 [http://irclogs.jackgrigg.com/irc.freenode.net/buildbot/2014-10-29#i_3482806
 IRC] suggestion.

 (This is just not to forget to do it).

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Ticket URL: <http://trac.buildbot.net/ticket/2989>
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