Add support of Alpine based environment

[iVegas]

In general, Alpine contains the ps command but not supports the -p option.
More detailed info can be found here - wodby/php#130

Expected Behavior

The managing of the child processes should just work.

Current Behavior

Currently each child process of the worker "freeze" after performing.

Possible Solution

Use PHP context to figure out if a child process exists in the returned list of the processes from executing ps -o command.

Context

Some of the provided docker php images do not contain the version of the ps command which compatible with the one used in the php-resque library.