Proxy: add special case for `container()` in scalar-mode gather2D-behavior

Required to allow eg. v3 tracks to be the target in a SOAcollections using an indices2D_field, eg.

struct ChildIndexArray : LHCb::Event::indices2D_field< LHCb::Event::v3::Tracks const , 2, 8 >{};

Should go with Rec!2749 (merged)