It should be possible to specify an AGENT_SECRET_FILE to be compatible with Docker Secrets

[inieves]

To be more compatible with Docker Swarm secrets, which are files passed into the swarm not via compose files, it would be more secure and closer to convention if the agent secret could be given as a path to a file (which would be a path to a docker secret).

This seems to be currently not possible, at least according to the documentation at: https://portainer.readthedocs.io/en/stable/agent.html#configuration

This change will involve both the agent and Portainer codebases