Docker improvements

[byroncollins]

• Added documentation for running the application as a docker container
• reorganised dockerfile to improve cache utilisation
• Moved the application base image to a non privileged container (running in a non-root user and on a unprivileged port). Most Enterprise kubernetes distributions restrict containers by default from running as root and using privileged ports.
• Exposed port changed from 80 to 8080
• Modified github actions to build new container when a new git tag is created( new release)
• Updated ASzc/change-string-case-action to v6 to resolve issue with action running unsupported nodejs version.

Would recommend also updating the overview on https://hub.docker.com/r/ckabalan/visualsubnetcalc.

closes #9