Reorganize Dockerfiles / images

[jtagcat]

https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds

• As part of build, the api image should include the result of the current frontend image.
• might be a good idea to split api and celeries
• celeries should have a base image, and add to it with 2 separate images

  FROM docker.io/JefferyCA/spleeter-web-celery-api
  ENTRYPOINT ["celery-fast-entrypoint.sh"]
  

  ^ would be JefferyCA/spleeter-web-celery-fast