Support docker secrets (i.e. secrets provided by a file path)

[MoralCode]

Docker's docs for passing environment variables in compose files say

Don't use environment variables to pass sensitive information, such as passwords, in to your containers. Use secrets instead.

it seems like this makes secrets available via a path, such as /run/secrets/<secret name> within the container, rather than via env variables (which docker claims are easier to compromise)

In order to use this with hebbot, I think hebbot would need to support something like the _FILE environment variables (i.e. BOT_PASSWORD_FILE, which docker says

are a convention used by some images, including Docker Official Images like mysql and postgres.