This would be a very bad idea, in my opinion.

Diesel has a shelf life of around six to 12 months before it starts to degrade. After six or seven years, I'd guess that the diesel has become contaminated with water and bacteria. You should not put this gunk into your car's engine.

As for disposal, you'll need to contact your local recycling centre. Not all sites will accept it but they may know of the nearest one that does.