[Buildbot] #2993: Better document the multi-container Docker solution

[Buildbot trac]

#2993: Better document the multi-container Docker solution
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Reporter:  Ben     |      Owner:
    Type:  task    |     Status:  new
Priority:  minor   |  Milestone:  undecided
 Version:  master  |   Keywords:  docker
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 We now have under contrib/Dockerfiles two Docker files that demonstrate
 how to run buildbot under separate Docker environment.

 They should be better mentioned in the  manual.

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Ticket URL: <http://trac.buildbot.net/ticket/2993>
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