I claim the ornamental design for the butter churn container, as shown and described.
FIG. 1 is a perspective view of a butter churn container, showing my new design.
FIG. 2 is a front view of the butter churn container, the left side, right, and rear views being identical.
FIG. 3 is a top view of the butter churn container.
FIG. 4 is a bottom view of the butter churn container; and,
FIG. 5 is a perspective exploded view of the butter churn container, shown with a representative plunger for use with the butter churn container. The plunger is illustrated in phantom lines and forms no part of the claimed invention.