dist: add Dockerfile for docker builder container

This PR adds a Dockerfile that can be used to build a docker[1] build container for RIOT.

It enables any modern Linux system to create a build environment / toolchain with just two  commands:

```# apt-get install docker.io``` (or the equivalent on your distro)
```# docker build -t riotbuild - < dist/Dockerfile```
This will create a Docker container based on ubuntu LTS with all toolchains (package list stolen from travis.py) installed.

It can then be used to run the testsuites locally, e.g.,
```# docker run -i -t -v $(pwd):/data/riotbuild riotbuild ./dist/tools/compile_test/compile_test.py```

It might even be useful if compiling/installing a native toolchain is impossible/too complicated/$(other_reason).

[1] https://www.docker.com/