Publish a docker file for building the Mystikos base container

[jxyang]

We publish the base container as part of the Mystikos release, which tends to occur every few months. Users who build their applications on top of the base container are likely to miss the interim CVE fixes. We should publish the docker file for users to build a Mystikos base container themselves.

[CyanDevs]

We have the following available in our repository which should cover this use-case:
Dockerfile for Mystikos base container: https://github.com/deislabs/mystikos/blob/main/.jenkins/docker/base/Dockerfile
Build script for the Dockerfile: https://github.com/deislabs/mystikos/blob/main/.jenkins/docker/base/build.sh

An example of using the build script to build a Ubuntu 18.04 Mystikos base container would be:

chmod +x build.sh
mkdir build
cd build
../build.sh -m "0.9.0" -o "latest" -u "18.04" -t "myTag"

Note that -o can take specific tags (e.g. 2022.06.1626 ) as well. Open Enclave base container images are hosted on Azure Container Registry now, and versions can be found at: https://github.com/openenclave/openenclave/blob/master/DOCKER_IMAGES.md and I will make a PR to update this shortly.

Any feedback on these would be appreciated.