Update Dockerfile to use the correct busybox:latest identifier

Also requires some tests to be updated which relied on behavior
of a busybox image that wasn't actually "busybox:latest"; meaning these
tests were unable to be verified/run against a real busybox:latest image
on a daemon.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)