Update the build workflow for Docker images so that images per service are build concurrently

[nicomiguelino]

Overview

• Builds are currently divided by platform (Pi 1, 2, 3, 4 & x86).
• The build process can further be optimized by splitting image builds (per service) into separate jobs (via matrices), where service $$\epsilon$$ {server, celery, redis, websocket, nginx, viewer, wifi-connect}.
• This means that there will be a total of 5 x 7 = 35 concurrent image build jobs running, in theory.
• GitHub Actions has a 256-job limit, as stated in this documentation.

[vpetersson]

Yes, but i think we would gain a lot more by just optimizing caching. We could do both of these in parallel of course, but it should be highly cacheable.