Release 0.3b: version bump for docker image

Another year passed by, time to update the image. No code changes this time.

• Version bump of the Docker image to Alpine Linux 3.21.2

Release 0.2b: code optimizations, improved documentation, version bumps

After two years of operations it was time to do some maintenance on this repository:

• The code has been optimized at several points.
• Documentation has been improved, both on formatting and content. It is now more complete (e.g. before nothing was mentioned about how to obtain a InfluxDB token).
• Versions have been updated in Dockerfile, the image is now based on Alpine Linux 3.19. Also InfluxDB versions in different files have been updated.