new docker build supporting edo and static binary

Proof of concept of building static binaries in Docker w/ reasonable caching and supporting Edo.

I didn't actually switch to Edo in this commit, but if rebased over Edo upgrade, it works To run docker build(and tests), creating a container, and extracting the ligo binary:

docker build . -t ligo:alpine
docker container run -itd  --name ligo-container ligo:alpine
docker cp ligo-container:/root/ligo .

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TODO (not urgent I guess) What do we do about docker-large ? see #205 (closed)

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The .gitlab.yml stages has changed a little bit (added comments in the stages declarations

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nix build has been trimmed of all the "code building" targets but is still used for the changelog, building the website...

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relates to #1094 (closed)