Various minor changes

[shadow1runner]

I just figured that I made some changes a while back and I forgot to upstream them, mainly because some of which are quite opinionated, just like:

• building the dockerfile from the current branch might be better than cloning your main branch from scratch
• exiting upon being unable to connect to MQTT instead of silently gulping

Please check out the changes and let me know what you think; I can also split the PR into multiple if preferred.
Also there's no need to merge each commit -- thanks for your continuous efforts and great work :-)

[AnotherDaniel]

LGTM - not 100% sure about the quit-on-failure behavior for the mqtt sink, guess preferences on this one might go both ways depending on the larger system context. But let's try with this approach.