Add docker launcher support?

[goodboy]

We've got the classic PopenRunner but what about a back-end that launches commands using a SIPp docker image?

There's already a well documented/maintained v3.5.0 image and the only tricky part will be passing -v and -p args to docker run/create. I think this might be a nice add-on to aid those who want to avoid the pains of building SIPp from source.

Opinions @vodik @wdoekes?