docker: master/worker: fix issue with latest stable sqlalchemy

docker: master/worker: fix issue with latest stable sqlalchemy

Fixes following issue with sqlalchemy version 1.4.0 and too relaxed
dependecies `sqlalchemy>=1.2.0` in buildbot:

 Traceback (most recent call last):
   File "/usr/local/bin/buildbot", line 8, in <module>
     sys.exit(run())
     from buildbot.db import enginestrategy
   ...
   File "/usr/local/lib/python3.7/dist-packages/buildbot/db/enginestrategy.py", line 154, in <module>
     class BuildbotEngineStrategy(strategies.PlainEngineStrategy):
 AttributeError: module 'sqlalchemy.engine.strategies' has no attribute 'PlainEngineStrategy'

References: https://github.com/buildbot/buildbot/issues/5911
Signed-off-by: Petr Štetiar <ynezz@true.cz>