Use default nginx image and allow versioning
Use default nginx image and allow versioning

With this change you can now pass the build argument VERSION and the
output will be written to the subfolder /$VERSION in the final output.

This will allow us to write different versions of the documentation to
various subfolders; which makes sure we can have a folder structure
/latest, /2.9, /3.0, etc on docs.phpdoc.org

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