feat: reduce image size by 100MB using multi-stage dockerbuild, introduce using a user

[jlongman]

Introduce a multi-stage build - reduces size from ~160MB by ~100MB down to 60MB.

• use the --user installation which puts the installation into $HOME/.local.
• change the entrypoint to reference as so

Use a user in the final container - best practices, security.

• move from /root/.local to /home/appuser/.local

note, I've only tested docker run --rm -it -ePYHQ_USER -ePYHQ_PASSWORD from the cli as I don't have a mqtt setup atm.