Add instructions for setting AZURIOM_GAME in INSTALL.md

[thomasbouillon]

Description of your suggestion

Hi,

After a fresh installation with the docker instructions from INSTALL.md, the site looked fine, but after going through the code I discovered that we should have set a AZURIOM_GAME=... in the dotenv.

Would be nice to add a section in the INSTALL.md regarding this parameter. Otherwise, you get stuck because some parts of the administration panel depends on your chosen game.

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