Optimize Dockerfile.

This optimizes the Dockerfile by;

- Combining RUN statements so that files are removed in the
  same layer as they are added.
- Removing the downloaded .tar.gz of the docker-gen binary
  after expanding
- Adding `--no-install-recommends` (but explicitly installing
  ca-certificates)
- Replacing `ADD` with `COPY` (recommended if no unpacking is
  required)

Also added a `.dockerignore` file to prevent the `.git` directory
and README.md being added to the image.

These changes reduce the size of the image with 34 MB (was 268.4 MB,
now 233.9 MB), and results in less layers being produced.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>