FIG. 1 is a top, front and right side perspective view of the stackable beverage container showing my new design; FIG. 2 is a bottom, front and right side perspective view thereof; FIG. 3 is a front elevational view thereof; FIG. 4 is a rear elevational view thereof, both sides and the rear being the same; FIG. 5 is a top, plan view thereof; and, FIG. 6 is a bottom, plan view thereof.