Experiment with using Docker Secrets (but not in Swarm mode)

Following on from conversations with [pwalsh](https://github.com/pwalsh) I will investigate how using Docker secrets (while **not** in Docker Swarm mode) give's any advantage over the PR: https://github.com/ckan/ckan-docker/pull/76

For reference:
https://docs.docker.com/compose/use-secrets/
https://gist.github.com/brianjbayer/966769aa0b00bb95b266827862dd511e
https://github.com/docker/compose/pull/4368

Start by reviewing PR #76, the Docker Compose secrets documentation, the referenced gist, and Docker Compose PR #4368. Compare the proposed non-Swarm secrets approach with the existing setup and document whether it provides a concrete advantage. Done means the investigation has a clear conclusion and any resulting change is reproducible.

Written by the indexing model from the issue text.