Fix #200 debian package

• Script to build deb packages using Docker.
• Build deb packages in GitLab CI.
• Two services (mirror and smith) for Duniter.
• A service and systemd timer for the distance oracle.
• Dev documentation to build deb packages.
• User documentation to install and run a node.

One hypothesis I made for the sake of simplifying user configuration is that the chain ID equals the chain name. This may not be valid for testing, but it is valid for live chains. This way, we can generate the oracle distance base path using the node and chain name paths.

docker/build-deb.Dockerfile

+FROM rust:slim-buster
+
+WORKDIR /root
+
+RUN apt-get update && apt-get upgrade -y && \
+    cargo install cargo-deb
+
+RUN cargo build -Zgit=shallow-deps --release \
+    && cargo deb --no-build -p duniter \
+    && cargo deb --no-build -p distance-oracle \