Dockerize the build

[bassosimone]

I would like to dockerize the build. It will work as follows:

1. there will be a top level Dockerfile that will create a minimal Debian stable container

2. the user will be instructed to create the container and then run it

3. the container shall be minimal

4. the download step shall also download a specific version of the NDK

5. all the other steps will be equal except that we will know the location of the NDK given that we download it

6. of course all build commands will be run in the container, however the smoke testing will need to be run outside it

I think this provides these advantages: (1) we pin to a specific build environment and version of the NDK; (2) we can run the same build regardless of the OS that we use; (3) the build machine may be beefier than my old laptop and hence most likely faster.