Fix cross when stdin is not a TTY

When stdin is not a TTY, cross fails because it still passes `-i` to
`docker run`, which makes Docker fail like so:
```
the input device is not a TTY
```
Apparently GitHub Actions doesn't have a TTY as stdin, so this is the
fix to make cross work with GitHub Actions.

src/docker.rs

@@ -59,10 +59,10 @@ pub fn register(target: &Target, verbose: bool) -> Result<()> {
         "apt-get update && apt-get install --no-install-recommends -y \
             binfmt-support qemu-user-static"
     };
+
     docker_command("run")
         .arg("--privileged")
         .arg("--rm")
-        .arg("-i")
         .arg("ubuntu:16.04")
         .args(&["sh", "-c", cmd])
         .run(verbose)
@@ -163,10 +163,13 @@ pub fn run(target: &Target,
 
     if atty::is(Stream::Stdout) && atty::is(Stream::Stderr) {
         docker.arg("-t");
+        if atty::is(Stream::Stdin) {
+            docker.arg("-i");
+        }
     }
 
     docker
-        .args(&["-i", &image(toml, target)?])
+        .arg(&image(toml, target)?)
         .args(&["sh", "-c", &format!("PATH=$PATH:/rust/bin {:?}", cmd)])
         .run_and_get_status(verbose)
 }