fix: update docker mem usage to match latest docker CLI

[101100]

Required for all PRs:

resolve: #10518

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

This updates the Docker input plugin memory statistics. Specifically, the usage measurement in docker_container_mem is updated so that it will match the value given by the Docker CLI tool version 20.10+. The calculation used in the CLI tool can be seen here. In Docker 20.10, the memory stats available changed and the cache field is no longer present. Instead, depending on the version of cgroups used, either total_inactive_file or inactive_file will be present.

I've added a test of this functionality. The values chosen are odd because in order for the test to pass, the percentage calculation had to match exactly despite floating-point inaccuracies.

[telegraf-tiger]

Thanks so much for the pull request!
🤝 ✒️ Just a reminder that the CLA has not yet been signed, and we'll need it before merging. Please sign the CLA when you get a chance, then post a comment here saying !signed-cla

[101100]

!signed-cla

[powersj]

Thank you very much for this fix!