Long render queries block the heartbeat of the bot

[behackl]

          It seems as if long (>30 sec?) render queries block the heartbeat of the bot; the communication with docker should be made asynchronous if possible. (There is https://aiodocker.readthedocs.io/en/latest/index.html which could be a suitable replacement for the docker package.)

Originally posted by @behackl in #38 (comment)