[docker] Add LFS_START_SERVER option to control git-lfs support

[slonopotamus]

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[slonopotamus]

To be honest, this is far from perfect. Ideally, I'd like

1. To be able to override any option in app.ini via environment variables without writing specific code for each of them
2. To be able to override any option via environment variables even when /etc/gitea/conf/app.ini already exists.

But for what I am trying to achieve (start Gitea in Docker with LFS without writing app.ini), exposing LFS_START_SERVER through environment variables will be enough.

[zeripath]

See #350