Add Dockerfile

[Uinelj]

Adds both lean and full fledged Docker images to ease use of Ungoliant.
The lean one should only contain the binary (Bring your own model..)
The full fledged one contains it all but the kenlms (they are heavy).

TODO: Another image to do the downloading.
If possible make one that builds everything, and two dockerfiles that inherit from that first one and specializes into generation and downloading :)

[codecov]

Codecov Report

Merging #116 (f0ed95a) into dev (9234d13) will not change coverage.
The diff coverage is n/a.

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[Uinelj]

We should have

• A stage image (ungoliant-base or something along these lines)
• An ungoliant-download image that only does the downloading part
• An ungoliant-gen image that generates the dataset, with:
  • An ungoliant-gen:light one that requires you to provide blocklist, model and kenlms
  • An ungoliant-gen:full one that has everything we use internally. Probably big (~20GB)