Optionally avoids mounting /var/run/docker.sock

By default, drun always did -v /var/run/docker.sock:/var/run/docker.sock. Since this was causing problems in some cases, we added some verification to see if the socket is operational before mounting. Also, drun -D won't even try to mount the daemon socket.

Extras:

• use -A if you prefer alpine images (e.g. drun -NA will do docker mhart/alpine-node-auto:x.x.x instead of docker node:x.x.x)
• if XTRACE is set outside of drun, it effectively enables the -x option