Add ability to set the MTU size of the docker in docker container (#385)

* adding abilitiy to set docker in docker MTU size

* safeguards to only set MTU env var if it is set

api/v1alpha1/zz_generated.deepcopy.go

@@ -689,6 +689,11 @@ func (in *RunnerSpec) DeepCopyInto(out *RunnerSpec) {
 		*out = new(bool)
 		**out = **in
 	}
+	if in.DockerMTU != nil {
+		in, out := &in.DockerMTU, &out.DockerMTU
+		*out = new(int64)
+		**out = **in
+	}
 }
 
 // DeepCopy is an autogenerated deepcopy function, copying the receiver, creating a new RunnerSpec.