[83] Resolve "BUG: support all unicode characters"

Closes #83 (closed)

BUGFIX:

• SYMPTOMS: emoji and accented characters transmitted over signalboost as ��� characters
• CAUSE: signald was not properly utf-8 encoding socket messages because the docker container in which it was running had not had locale env vars initialized to uft-8 encoded variants
• FIX: update both signalboost.dockerfile and signald.dockerfile to use utf8-compliant locale configurations

side-effects:

• bump base docker containers from debian stretch to debian buster (current stable)
• bump node from 10.15.3 to 12.9.1 (current stable)
• bump jdk from 8 to 11 (current stable, 8 no longer supported on buster)