Fix crash in psycopg2 with docker image

[thopre]

Summary

Psycopg2 2.8 crashes when used with PostgreSQL 12 [1] which is now the
default version of the postgres:alpine docker image. This commit sets
the tag for the postgres image used by the LNT service to 11-alpine to
prevent this issue. Going forward the requirement for psycopg2 will need
to be bumped again to 1.3.16 or later but given how new is that release
it's better to fix the PostgreSQL version for now.

[1] https://github.com/sqlalchemy/sqlalchemy/issues/4463

Test Plan: cd docker && docker-compose build && DB_PWD=foobar
docker-compose up work successfully with this patch.

[thopre]

Landed as f34ad041dce4f3b55a013ab6e7d712349a2844a0

docker/docker-compose.yml

lnt:
     deploy:
       restart_policy:
         condition: on-failure
     ports:
       - "8000:8000"
 
   db:
     container_name: lnt-postgres
-    image: postgres:alpine
+    image: postgres:11-alpine
     environment:
       - POSTGRES_PASSWORD=${DB_PWD}
       - POSTGRES_USER=${DB_USER}
       - POSTGRES_DB=${DB_BASE}