Stop post start subshell ending up as a zombie

[NeilW]

foreground.sh: Double detach the post_start subshell to ensure it is
reaped properly by 'init' after it has completed and avoid a zombie
process hanging around.

Summary of changes

In Docker/systemd configs the current foreground script leaves an annoying zombie process hanging around. This change gets rid of that.

$ docker exec b4c3d7fce641 ps -ef
PID   USER     TIME  COMMAND
    1 root      0:00 /dev/init -- /bin/sh -c exec /opt/app/bin/${APP_NAME} foreground
    6 root      0:05 {beam.smp} /opt/app/releases/0.1.0/ant_farm.sh -Bd -- -root /opt/app -progname opt/app/releases/0.1.0/ant_farm.sh -- -home /root -- -boot_var ERTS_LIB_DIR /opt/app/lib -config /opt/app/var/sys.config -pa /opt/app/lib/ant_farm-0.1.0/consolidated -noshell -noshell -noinput -boot /opt/app/releases/0.1.0/ant_farm -name [email protected] -setcookie test_cookie -smp auto -mode embedded -extra --
  129 root      0:00 [bash]
  134 root      0:00 /opt/app/erts-10.3.5.1/bin/epmd -daemon
  138 root      0:00 erl_child_setup 1048576
  157 root      0:00 ps -ef

Licensing/Copyright

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related material intended to be compiled or integrated with the source code for Distillery
(the "Contribution"). My Contribution is licensed under the MIT License.

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[bitwalker]

Thanks!