Ensuring Git requirement are noted across different installation methods

[chinyeungli]

https://github.com/aboutcode-org/scancode-toolkit/blob/develop/docs/source/getting-started/install.rst?plain=1#L330
It indicates that Git needs to be installed before running git clone. We should probably include a similar note in the 'Installation via Docker' section, since it also uses on git clone.

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