The ornamental design for a dock for a keyboard, as shown and described.
FIG. 1 is a front perspective view of a dock for a keyboard according to our design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a rear perspective view thereof;
FIG. 9 is a rear view of a second embodiment thereof, the front, left side, right side, and bottom views are the same as those in the first embodiment;
FIG. 10 is a top view thereof; and,
FIG. 11 is a rear perspective view thereof.
Broken lines shown in the drawings illustrate portions of the dock for a keyboard and form no part of the claimed design.