Add source and fetcher for Docker containers

[amesgen]

Closes #33

Supports new entries like

[alpine]
src.container = "library/alpine"
src.include_regex = "3\\..*"
fetch.docker = "library/alpine"

Additional options include

docker.arch = "arm64"
docker.tlsVerify = false

Adds a dependency on nix-prefetch-docker.

[berberman]

LGTM, Thank you!