Add tty to exec

[lordbyron]

The docker run command always has tty support, but this plugin invokes most commands using exec, which currently does not allow tty (see https://github.com/jenkinsci/docker-workflow-plugin/blob/50ad50bad2ee14eb73d1ae3ef1058b8ad76c9e5d/src/main/java/org/jenkinsci/plugins/docker/workflow/client/DockerClient.java#L109)

An alternative would be to allow script writers to specify additional flags to the exec commands, the same way we can pass flags to inside.