Fix GRR docker version docs

[mike-bailey]

Added it so latest docker version is pulled, not two older seemingly random versions. Pulling the older version resulted in me having to pull latest before getting help on an issue, so I figured I'd fix that in the future.

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[mike-bailey]

I signed it!

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CLAs look good, thanks!

[mbushkov]

Thanks for the PR! The reason I asked you to use grr:latest in google/grr#499 is to check that the issue still persists in HEAD (since GetMRU was removed and replaced by an artifact). Normally we recommend to try the release version first, and switch to the HEAD version if needed (i.e. if latest features/changes/bugfixes are needed).

[mike-bailey]

Thanks for the reply! Shouldn't the release version be the latest docker version....?

[mbushkov]

Latest Docker version is GitHub's HEAD. It should work (it passes the automated tests) but doesn't go through a release process.

The release version (currently - 3.1.0.2) is released periodically and goes through a release process: it is tested with a more comprehensive integration testing suite and should be more reliable.