The ornamental design for an onioncontainer, as shown and described.
FIG. 1 is a top view of a onion container showing our new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a front elevational view thereof; and,
FIG. 6 is a rear elevational view thereof.
The broken lines shown in FIGS. 2, 5 and 6 are included for the purpose of illustrating environment of the design and forms no part of the claimed design.