Allow setting default Retention policy with Variable on Docker deployment

Having a variable such as DEFAULT_RETENTION_POLICY to set the number of days in the default retention policy would make it a lot easier for deployments where storage is a concern.

No file or test is named in the issue. Start by locating the Docker deployment's environment-variable handling and the code or configuration that creates the default retention policy. Done means a DEFAULT_RETENTION_POLICY value controls the default number of retention days for deployments where it is set, with existing behavior preserved when it is absent.

Written by the indexing model from the issue text.