Add readme docs for building 'Enterprise' containers in closed environments.

[vrogach2020]

Now default testcontainer Dockerfile works only with community version.
See

Customer wants to use enterprise version in a closed environment.

There are 2 scenarios of using enterprise version:

1. pack cartridge app via cartrdige pack docker .
   This command create a 'single' instance image.
   We need to add support for such container images to set up the cluster with multiple instances.

2. create Dockerfile template that includes enterprise bundle.
   Cartridge app is built via enterprise bundle and packed into docker image.
   The Dockerfile must somehow include sdk archive instead of using curl and installer.sh.

[vrogach2020]

After some research the option (2) was chosen.
I've fixed issues that blocked building and testing tarantool cartridge clusters in closed environment (#23 and #24).
Also created an example of Dockerfile that uses TE instead of community version.

To close the issue we need to add an example of using TE Dockerfile in readme.