Yes, you can buy alcohol and other liquid items at Qatar Duty Free and board a flight to the US provided the bottles/liquids are in a STEB bag. This is sealed and tamper proof and the only kind allowed for transport into the US. There was a time a few years ago that Qatar Airways would force you to check-in your hand bag containing your duty free bottle purchases (happened to me), but that is not longer the case, you are free to buy and carry with you in the STEB bag they provide to the gate and through security check at the gate.
See this: https://www.qatardutyfree.com/Allowances.aspx