Add metadata to Digital Ocean doc page

doc/install/digitaloceandocker.md

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-stage: none
-group: unassigned
+stage: Enablement
+group: Distribution
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
-type: howto
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 # Digital Ocean and Docker Machine test environment **(FREE SELF)**