[Buildbot] #2994: Don't link to the master branch from the doc.

[Buildbot trac]

#2994: Don't link to the master branch from the doc.
------------------------+-----------------------
Reporter:  Ben          |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  undecided
 Version:  0.8.9        |   Keywords:  docs
------------------------+-----------------------
 The docker turorial is instructing people to download the Dockerfile from
 the master branch of git.

 This is rather bad as the file and the doc can get pretty much out-of-
 sync.

 From Dustin
 (https://github.com/buildbot/buildbot/pull/1311#issuecomment-61504982):

   Regarding the unfortunate use of /latest/ in the link: it shouldn't be
 too difficult to build a :bb:src: Sphinx/docutils role that would link to
 the source on the proper tag (based on version in the Sphinx config). And
 we can always go back and manually edit the link in the
 https://github.com/buildbot/bbdocs repository for already-released
 versions.

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Ticket URL: <http://trac.buildbot.net/ticket/2994>
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