added support for Docker secrets.

[oaltun]

This PR adds support for Docker secrets.

All environment variables can also be passed as Docker secrets.

To use docker secrets, for each Docker secret XXX, in a docker stack file we need to have lines like:

services:
  fief:
    image: ...
    secrets:
      - XXX

secrets:
    XXX:
      external: true

We also need to add the secret in the command line like:

printf "my super secret password" | docker secret create XXX -

[codecov[bot]]

Codecov Report

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[frankie567]

@all-contributors add @oaltun for code

[allcontributors[bot]]

@frankie567

I've put up a pull request to add @oaltun! :tada:

[oaltun]

Great, thanks!