Follow hadolint best practices in order to have a docker build that is as reliable as possible.
- first best practice is to "Pin versions in apk add. Instead of `apk add <package>` use `apk add <package>=<version>`" (see [DL3018](https://github.com/hadolint/hadolint/wiki/DL3018))
- second best practice is to use "`--no-cache` switch to avoid the need to use `--update`"
