[Fixes #6728] SPCGeoNode Dockerfile build error - missing git

[ricardogsilva]

This PR modifies the spcgeonode Dockerfile for django to add git as an additional system dependency. This shall fix #6728

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[cla-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @ricardogsilva on file. In order for us to review and merge your code, please contact the project maintainers to get yourself added.