Update documentation to show example of pinning Agent version

[chynesNR]

Our documentation gives examples of how to install the Agent via Docker, but only how to always grab the latest version. This should be updated to show how to pin to a version as well.

Acceptance Criteria

Documentation site updated.

Estimates

S

[workato-integration]

https://new-relic.atlassian.net/browse/NR-179689