Docker executor in container launch

Note: I haven't embed the docker image or include Dockerfile and docker build as part of the build step yet, but this is the working code to put up review first.
Assuming you don't make 'value' const string& (even though I just asked that above), then you could do:

// "Escape" single quotes.
value = strings::replace(value, "'", "'\"'\"'");
value = "'" + value + "'";

cmd += " -e " + key + "=''" + value + "'";