Add support for volume mounts

[groja]

Expected Behavior

It would be useful to be able to support volume mounts rather than just bind mounts, as it lets you use a wider variety of volume drivers.

Current Behavior

Only bind mounts are supported

Possible Solution

The following change to the code seems to work (I'm new to Go so feel free to suggest a better way of formatting it):
Old:

if len(parts) == 2 {
			mountVal := mount.Mount{
				Source: parts[0],
				Target: parts[1],
			}

			spec.TaskTemplate.ContainerSpec.Mounts = append(spec.TaskTemplate.ContainerSpec.Mounts, mountVal)
		}

New:

if len(parts) == 2 {
                        mountVal := mount.Mount{}
                        if strings.HasPrefix(parts[0], "v:") {
                                mountVal = mount.Mount{
                                        Type: mount.TypeVolume,
                                        Source: strings.TrimLeft(parts[0], "v:"),
                                        Target: parts[1],
                                }
                        } else {
                                mountVal = mount.Mount{
                                        Source: parts[0],
                                        Target: parts[1],
                                }
                        }
                        spec.TaskTemplate.ContainerSpec.Mounts = append(spec.TaskTemplate.ContainerSpec.Mounts, mountVal)
                }

Then, if I want to use a named volume pass in:

-m "v:volumeName=/mnt/somewhere"

Context

I want to use JaaS with persistent volumes created by the Docker Swarm Cloudstor drivers.

Your Environment

• Version used: master
• Environment name and version (e.g. Docker 1.13): Docker CE 19.03.5
• Server type and version: Ubuntu 18.04LTS