fix panic in docker driver for newer docker version (18.09.7+) (#813)

cmd/docker-driver/driver.go

@@ -12,6 +12,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/docker/docker/api/types/backend"
 	"github.com/docker/docker/api/types/plugins/logdriver"
 	"github.com/docker/docker/daemon/logger"
 	"github.com/docker/docker/daemon/logger/jsonfilelog"
@@ -124,6 +125,9 @@ func consumeLog(lf *logPair) {
 		msg.Line = buf.Line
 		msg.Source = buf.Source
 		if buf.PartialLogMetadata != nil {
+			if msg.PLogMetaData == nil {
+				msg.PLogMetaData = &backend.PartialLogMetaData{}
+			}
 			msg.PLogMetaData.ID = buf.PartialLogMetadata.Id
 			msg.PLogMetaData.Last = buf.PartialLogMetadata.Last
 			msg.PLogMetaData.Ordinal = int(buf.PartialLogMetadata.Ordinal)