Merge pull request #174 from priyawadhwa/shell

Add shell command to commands.go

pkg/commands/commands.go

@@ -63,6 +63,8 @@ func GetCommand(cmd instructions.Command, buildcontext string) (DockerCommand, e
 		return &VolumeCommand{cmd: c}, nil
 	case *instructions.StopSignalCommand:
 		return &StopSignalCommand{cmd: c}, nil
+	case *instructions.ShellCommand:
+		return &ShellCommand{cmd: c}, nil
 	case *instructions.MaintainerCommand:
 		logrus.Warnf("%s is deprecated, skipping", cmd.Name())
 		return nil, nil