source tmate binary from docker image

command/daemon/config.go

@@ -106,6 +106,11 @@ type Config struct {
 		Config string `envconfig:"DRONE_DOCKER_CONFIG"`
 		Stream bool   `envconfig:"DRONE_DOCKER_STREAM_PULL" default:"true"`
 	}
+
+	Tmate struct {
+		Enabled bool   `envconfig:"DRONE_TMATE_ENABLED" default:"true"`
+		Image   string `envconfig:"DRONE_TMATE_IMAGE"   default:"drone/drone-runner-docker:latest"`
+	}
 }
 
 // legacy environment variables. the key is the legacy

command/daemon/daemon.go

@@ -141,6 +141,10 @@ func (c *daemonCommand) run(*kingpin.ParseContext) error {
 				CPUSet:     config.Resources.CPUSet,
 				ShmSize:    config.Resources.ShmSize,
 			},
+			Tmate: compiler.Tmate{
+				Image:   config.Tmate.Image,
+				Enabled: config.Tmate.Enabled,
+			},
 			Environ: provider.Combine(
 				provider.Static(config.Runner.Environ),
 				provider.External(