Fix: Broker's Dockerfile won't build

[ysbaddaden]

• Debian Jessie was moved to archive.debian.org (long deprecated)
• APT keys expired in June 2020

[ysbaddaden]

Some upgrade notes: if we could update to Erlang 18, we'd be able to move on to Debian Stretch (as the main Ruby image) which was last built 9 months ago (instead of 4 years ago)... same for Erlang 19 (Stretch, 9 months ago) while Erlang 20 was last built 3 days ago (Buster) :+1:

• https://hub.docker.com/layers/library/erlang/18.3.4.11/images/sha256-5cf098f90738e23af2f820f084067b8582789c8b68f1b882dc16e6f4adb39721?context=explore
• https://hub.docker.com/layers/library/erlang/19.3.6.13/images/sha256-36a857ec769b975d591d874aec344ea554a18a0191d7901cb7966d9bd5b1914c?context=explore
• https://hub.docker.com/layers/library/erlang/20.3.8.26/images/sha256-c31845304a0b7102f2c0a01faa2e2c5cde4203cec25b86129411f2e85c22b97e?context=explore

[ysbaddaden]

I tried to build an image from erlang:18.3. It doesn't hurt to try, but poirot fails to build because it triggers the one deprecation in ERTS from Erlang 18 :laughing:

The deprecation shouldn't fail the build (it's backward compatible), but we treat warnings as errors in both verboice/broker and poirot.

[matiasgarciaisaia]

Poirot is dead, so let's remove it.