Create Docker container images of example applications

[ynamiki]

A future release of Intel SGX may introduce a new feature to support for running SGX applications inside Docker containers. The feature described in the What's New section in Intel SGX 2.8 Release Note but it seems not to be included in this release and is still an ongoing issue: intel/linux-sgx#486.

Once the feature is released, we will consider providing container images of our example applications so that users can try our code with minimum setup.

[ynamiki]

I noticed Intel SGX Linux 2.9.1, which was released in early April brings some documents about Docker even there are no mentions in the release note:
https://github.com/intel/linux-sgx/tree/sgx_2.9.1/docker/build

I'll take a look the document to consider whether we can use it.

Note that, the release updates system requirements; now we need Ubuntu 18.04, and Ubuntu 16.04 is dropped.

[ynamiki]

Done in #193 for the simulation mode. Needs more work for the HW mode.