Dockerfile: don't ADD host build folders

`ADD .` will potentially add (at least one) `build` folder when used
in a CMake project. Since CMake `build` folders contain cached path
information a copy of it will make the compilation fail. Therefore,
we use `ADD` to explicitly select data needed for compilation and other
tests. The Docker image size is cut into half as a side effect.

Also, `rm -rf build` is removed since this command does not make any
sense to be executed in a folder that has just been created.
Merge pull request #500 from toxeus/fixDocker

Dockerfile: don't ADD host build folders
Removed unused message buffers in connection.
Removed linking to ZMQ in ext and added it for client and server libraries.