CI/Docker: only publish images for active networks

The only useful docker images are:

• the latest release build of master branch for other tools development
• the latest ones for a given live network (gdev / gtest /g1) with embedded chainspecs (bootnodes and so on)

We should:

• disable publishing debug docker images
• build in release mode with relevant feature (ex --features gtest --no-default-features)
• publish with an image name containing the target network (ex duniter/duniter-v2s-gtest:latest)

By using GitLab registry we could differentiate channels.

See https://forum.duniter.org/t/release-new-image-with-gdev5-chainspecs/10723/46 for discussion on the forum.

The live network will always be in a dedicated branch, sometimes rebased on master for client upgrades. Updates will most likely only touch to bootnodes (and code substitutes but hopefully not).

MR: !191 (merged)