Can use Docker secrets without Docker Swarm

[chughts]

The statement

Docker secrets are only available via Docker Swarm services, hence to create a secret using docker, Docker Swarm must be used.

in docs/developer-config.md is not true.

It is also possible to use compose with both Docker and Podman the secrets end up in the same place /run/secrets. Though this does necessitate a docker-compose.yaml file.