Check if stdin is a tty and add docker flag accordingly (#34)

Fixes #33

TODO: do the same thing in powershell for the windows provider

pkg/providers/docker_unix.go

@@ -9,7 +9,9 @@ const (
 	// TODO: there might be a way were users can configure a template for the
 	// actual execution since some CLIs require some other folders to be mounted
 	// or networks to be shared
-	sh = `docker run --rm -i -t -v ${PWD}:/tmp/cmd -w /tmp/cmd %s:%s "$@"`
+	sh = `
+	termflag=$([ -t 0 ] && echo -n "-t")
+	docker run --rm -i $termflag -v ${PWD}:/tmp/cmd -w /tmp/cmd %s:%s "$@"`
 )
 
 // getImageName gets the name of the image from the image repo.