FIG. 1 is a top, front and right side perspective view of a stackable beverage container showing my new design; FIG. 2 is a bottom, front and right side perspective view thereof; FIG. 3 is a front devotional view thereof; FIG. 4 is a side devotional view thereof, both sides being the same; FIG. 5 is a rear elevational view thereof; FIG. 6 is a top plan view thereof; and, FIG. 7 is a bottom plan view thereof.