Because there is no re-exec operation, there is no need to use
environment variables to pass parameters between bootstrap and
runelet, use global variables instead of environment variables.
Signed-off-by: NTianjia Zhang <tianjia.zhang@linux.alibaba.com>

Signed-off-by: Njack.wxz <wangxiaozhe@linux.alibaba.com>

It was intended to have fds without close-on-exec with the side
effect of dup(), but acutally all fds staged are already
close-on-exec clear. Thus dup() makes extra duplications of fds
passed to init-runelet.
Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>

inclavare-containers is a set of tools for running trusted
applications in containers with the hardware-assisted enclave
technology. Enclave, referred to as a protected execution
environment, prevents the untrusted entity from accessing the
sensitive and confidential assets in use.

Currently, inclavare-containers consists of two core components:
rune and enclave runtime.

rune is a CLI tool for spawning and running enclaves in containers
according to the OCI specification. The codebase of rune is
a fork of runc, so rune can be used as runc if enclave is not
configured or available.

Enclave runtime is the backend of rune, which is responsible
for loading and running applications inside enclaves. The
interface between rune and enclave runtime is Enclave Runtime PAL
API, which allows invoking enclave runtime through well-defined
functions. The software for confidential computing may benefit
from this interface to interact with OCI runtime.

Additionally, this commit includes additional information about the
use of inclavare-containers.
- Run sample enclave runtime skeleton with rune
- Run enclave runtime Occlum with rune

See README.md for more details.
Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>
Signed-off-by: NXiaozhe Wang <wangxiaozhe@linux.alibaba.com>
Signed-off-by: NYilin Li <YiLin.Li@linux.alibaba.com>