Recommend tini when running local agent in docker

[jcrist]

Adds a note to the docs recommending using an init process like tini
when running the local agent inside a docker container.

Fixes #2418.

• adds new tests (if appropriate)
• add a changelog entry in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[codecov]

Codecov Report

Merging #2925 into master will not change coverage.
The diff coverage is n/a.