keactrl does not work on Alpine Linux: ps problem

The problem is the default ps command on Alpine Linux is not a POSIX one and does not support the -p argument:

BusyBox v1.30.1 (2019-06-12 17:51:55 UTC) multi-call binary.

Usage: ps [-o COL1,COL2=HEADER]

Show list of processes

	-o COL1,COL2=HEADER	Select columns for display
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Solved by installing a full ps command from the procps package.