Storing 50-100 gallons of gasoline in anything other than a tank specifically designed for that purpose is insane, dangerous, and in most places illegal. 5 Gallon metal tanks are intended for the temporary storage of fuels, and even with a rotation plan, you'll be storing a lot of fuel in a garage, a practice that represents a serious risk of fire and explosion. 5 Gallon tanks do not have the best vapor seals, they can and do leak fumes. Tanks designed to store gasoline are not cheap, for a reason. What's more, if you intend to use 100 gallons as a hedge against inflation, that's hardly going to be worth the effort. I've worked garage fires, it's bad enough with all the other stuff in there, throw in 100 gallons of gasoline and you've just made a bad problem much worse. Large gasoline tanks are underground or outdoors for a good reason - to prevent really bad fires.

The economics of a 100 gallon futures market are also questionable.
If you buy fuel at $4.25 a gallon, and you need 20 gallons a week, you're still going to be paying $4.25 a gallon, and then $4.75 and then $5.00 and so on to replenish your supply as you use it. You're not avoiding any expenses, you're just delaying them a week or so. Serious futures markets are contracts to purchase at a set price at a point in the future.

If you intend to store fuel for a while, for emergency use, the standard method is the metal 55 gallon drum: http://www.valumotion.com/index.php/cPath/700_4442_790_8949
You'll need to stabilize the fuel, a simple additive you can buy almost anywhere they sell gas-powered tools. You'll also need a gasoline rated pump to get the fuel out of the drums.
Finally, you'll need a way to FILL the drums in the first place, and handle them full. You can go back and forth with 5 gallon jugs to your drums. You'll need to be careful of static electricity when filling with 5 gallon cans.
Or, you can get a truck with a liftgate and fill the drums up at a gas station that is willing to look the other way while you fill them up (55 gallon drums would be considered an "unapproved" container in most places, and thus illegal to fill).
All in all, gas storage is tricky.