This commit provides a modified Go runtime in Docker image.
Now we can build a Go program using `occlum-go`, then run it
in SGX enclaves by Occlum.
The Golang demo demonstrates how to build and run a web server
program written in Go.
This demo consists of
1. An user-friendly library for getting Intel Attestation Service (IAS) report.
2. A application that uses this library to get IAS report.
This demo requires obtaining service provider certificate/ID from Intel.