refactor spoke-entrypoint.sh
Add universal command pass-through support.

Can be arbitrary commands "bash, ls" or a predefined "entrypoint" using
the env var $SPOKE_CMD. Which will behave like the normal entrypoint/cmd
relationship that dockerfiles already use. Otherwise, when no arguments
given to `docker run` command, normal startup is done.
Combine checks and only do on first run.

This commit also makes $WHEEL_REPO checking passive. If unset, we give a
warning and do no further checking and setup of the configuration
process will be done instead of making it manditory for Spoke-detach
mode.
Typos and small bug fixes to command pass-through
Update README with spoke-detach and SPOKE_CMD info