We claim the ornamental design for a container base, as shown and described.
FIG. 1 is a top perspective view of a container base.
FIG. 2 is a bottom perspective view of the container base of FIG. 1.
FIG. 3 is a side view of the container base of FIG. 1, with the view from all sides being identical.
FIG. 4 is a cross sectional view of the container base of FIG. 1, shown independent of the unclaimed container portion to show aspects of the design not visible when combined;
FIG. 5 is a top view of the container base of FIG. 4; and,
FIG. 6 is a bottom view of the container base of FIG. 4.
The evenly broken lines are shown for the purpose of illustrating parts of the article that form no part of the claimed design. The dot-dash broken lines represent the boundaries of the claim and form no part thereof.