Accept a runtime parameter

DockerSlim doesn't work in the gVisor runtime due to its lack of fanotify(7) support tracked here: https://github.com/google/gvisor/issues/2555 DockerSlim fails in the gVisor runtime with the following error:

docker-slim[build]: info=cmd.startmonitor status=sent
docker-slim[build]: info=event.error status=received data=SensorError{Op:sensor.fanotify.Run/fanapi.Initialize,Kind:call.error,Wrapped:{Type=syscall.Errno,Info=function not implemented,Line:43,File:github.com/docker-slim/docker-slim/internal/app/sensor/monitors/fanotify/monitor.go}}
docker-slim[build]: state=exited version=linux|Transformer|1.31.0|508fee6f9b2aa4c3aa3920189837730c4147214a|2020-08-14_04:55:24AM

Making DockerSlim work without the use of fanotify sounds challenging and very uncool. Another solution would be to pass the --runtime flag to Docker so the user can specify a different runtime such as runc to perform DockerSlim actions.

The issue names no files or tests. Start by locating the CLI flag definitions and the code that invokes Docker, then trace how runtime options reach DockerSlim actions. Done means a user can provide --runtime, the selected runtime is passed to Docker, and coverage verifies the option without triggering the reported gVisor fanotify failure.