Auto merge of #43950 - redox-os:redox_docker, r=alexcrichton

Add x86_64-unknown-redox to build manifest target list

This should fix the issue of `x86_64-unknown-redox` not being available for `rustup target add`

Author: bors

src/tools/build-manifest/src/main.rs

@@ -94,6 +94,7 @@ static TARGETS: &'static [&'static str] = &[
     "x86_64-unknown-linux-gnu",
     "x86_64-unknown-linux-musl",
     "x86_64-unknown-netbsd",
+    "x86_64-unknown-redox",
 ];
 
 static MINGW: &'static [&'static str] = &[