Dream: ability to configure network visibility

[akauppi]

Adding this as a discussion starter.

I'd like to have a way to:

• limit the emulators to "offline only" mode

Currently, they eg. query and download newer emulator images but this is not according to a versioned approach of Docker images. I'd like to disallow this - and it can be done.

Also, I'd like to:

• allow access to certain white listed Internet domain names/IPs

This is not so important, but I think it would be useful.