Added standalone flag to run jobberrunner as non-root

[AndiDittrich]

First of all, thanks for jobber it's a great tool to handle cron tasks in a modern way!

---

By running jobberrunner directly within a container/docker i've experienced some problems with existing images because the directory /var/jobber/${USERID} has to be existent (as mentioned in your official dockerfiles).

Therefore i've added a standalone mode (-s flag) to jobberrunner which skips the ipc routines/initialization.

Additionally i've modified the execution order of the argument/flag processing because -v or -h won't work in case -u or -p hasn't be set.

A minimal dockerfile can now look like this:

FROM debian:latest

# user downgrade
USER www-data

# new entrypoint
ENTRYPOINT [ "/usr/local/bin/jobberrunner" , "-s", "/etc/cron.yaml" ]