Linux – Installed Go binary not found in path on Alpine Linux Docker

I've got a Go binary I'm trying to run on the Alpine Docker image.

This works fine for the Docker Go binary.

docker run -it alpine:3.3 sh
apk add --no-cache curl

DOCKER_BUCKET=get.docker.com
DOCKER_VERSION=1.9.1
curl -fSL "https://${DOCKER_BUCKET}/builds/Linux/x86_64/docker-$DOCKER_VERSION" -o /usr/local/bin/docker
chmod +x /usr/local/bin/docker
docker help
Usage: docker [OPTIONS] COMMAND [arg...]
...

However, for the Go binary I want to install.

RACK_BUCKET=ec4a542dbf90c03b9f75-b342aba65414ad802720b41e8159cf45.ssl.cf5.rackcdn.com
RACK_VERSION=1.1.0-beta1
curl -fSL "https://${RACK_BUCKET}/${RACK_VERSION}/Linux/amd64/rack" -o /usr/local/bin/rack
chmod +x /usr/local/bin/rack

rack help
sh: rack: not found

/usr/local/bin/rack help
sh: /usr/local/bin/rack: not found

ls -al /usr/local/bin/
total 43375
drwxr-xr-x    2 root     root          1024 Jan 11 18:10 .
drwxr-xr-x    8 root     root          1024 Jan 11 18:09 ..
-rwxr-xr-x    1 root     root      30222575 Jan 11 18:09 docker
-rwxr-xr-x    1 root     root      14190576 Jan 11 18:10 rack

which rack
/usr/local/bin/rack

I thought it might have something to do with this answer but I don't get the same error when running ldd.

ldd /usr/local/bin/rack
    /lib64/ld-linux-x86-64.so.2 (0x7fdd15cd0000)
    libpthread.so.0 => /lib64/ld-linux-x86-64.so.2 (0x7fdd15cd0000)
    libc.so.6 => /lib64/ld-linux-x86-64.so.2 (0x7fdd15cd0000)

Any idea with this installed Go binary is not found in path on Alpine Linux Docker?