Evacuate it with a high vacuum pump running outdoors with a blower blowing the propane away from the pump's motor. When you reach a really good vacuum, say, 500 microns as indicated by a thermistor gauge, break the vacuum. All residual propane has been boiled to a gas state (provided the container no longer has any cold spots) and will no longer leech out of the metal. The concentration of propane is now 0.07% (30 times leaner than the lower flammability limit) and the mixture is completely non-flammable.

The key is that propane is only explosive at concentrations between 2% and 10%. Below 2% you can blow it right into a flame and it won't burn. Above about 10% concentration in air, it will not burn either... but it will further dilute in air and become an explosive mixture.