Add "memory_mb" option to Nomad docker driver

nomad_docker_memory_limit.patch

diff --git a/client/driver/docker.go b/client/driver/docker.go
index 0dacbac..ca1fe20 100644
--- a/client/driver/docker.go
+++ b/client/driver/docker.go
@@ -130,10 +130,11 @@ type DockerDriverConfig struct {
 	Interactive      bool                `mapstructure:"interactive"`        // Keep STDIN open even if not attached
 	ShmSize          int64               `mapstructure:"shm_size"`           // Size of /dev/shm of the container in bytes
 	WorkDir          string              `mapstructure:"work_dir"`           // Working directory inside the container
 	Logging          []DockerLoggingOpts `mapstructure:"logging"`            // Logging options for syslog server
 	Volumes          []string            `mapstructure:"volumes"`            // Host-Volumes to mount in, syntax: /path/to/host/directory:/destination/path/in/container
+	MemoryMB         int64               `mapstructure:"memory_mb"`          // Hard memory limit for docker container: 0 - use value from resources, -1 - no limit
 }
 
 // Validate validates a docker driver config
 func (c *DockerDriverConfig) Validate() error {
 	if c.ImageName == "" {
@@ -321,10 +322,13 @@ func (d *DockerDriver) Validate(config map[string]interface{}) error {
 				Type: fields.TypeArray,
 			},
 			"volumes": &fields.FieldSchema{
 				Type: fields.TypeArray,
 			},
+			"memory_mb": &fields.FieldSchema{
+				Type: fields.TypeInt,
+			},
 		},
 	}
 
 	if err := fd.Validate(); err != nil {
 		return err
@@ -669,11 +673,18 @@ func (d *DockerDriver) createContainerConfig(ctx *ExecContext, task *structs.Tas
 
 	if driverConfig.WorkDir != "" {
 		config.WorkingDir = driverConfig.WorkDir
 	}
 
-	memLimit := int64(task.Resources.MemoryMB) * 1024 * 1024
+	var memLimit int64
+	if driverConfig.MemoryMB == 0 {
+		memLimit = int64(task.Resources.MemoryMB) * 1024 * 1024
+	} else if driverConfig.MemoryMB < 0 {
+		memLimit = 0
+	} else {
+		memLimit = driverConfig.MemoryMB * 1024 * 1024
+	}
 
 	if len(driverConfig.Logging) == 0 {
 		if runtime.GOOS != "darwin" {
 			d.logger.Printf("[DEBUG] driver.docker: Setting default logging options to syslog and %s", syslogAddr)
 			driverConfig.Logging = []DockerLoggingOpts{