http.NewRequestWithContext will only set the right value for
Content-Length if the input is *bytes.Buffer, *bytes.Reader, or
*strings.Reader [0].
Since os.Stdin is an os.File, POST requests made with the `nomad
operator api` command would always have Content-Length set to -1, which
is interpreted as an unknown length by web servers.
[0]: https://pkg.go.dev/net/http#NewRequestWithContext
