Improve the Docker image

Improve the Docker image

1. Bump Node version to 6.9 - the latest LTS
2. Include the `cpio` binary, used during building
3. Ignore node_modules and .git directories for faster builds
4. Run the tests and build during the image build, to make sure the image is not built if these break
5. Make npm run dev the default command
6. Add an entrypoint to automatically install Node modules if they do nott exist

Signed-off-by: Antonis Kalipetis <akalipetis@gmail.com>