Expanding config files

[rsteube]

Would be nice to be able to use environment variables in the traefik.yml via expansion as combining the different configuration options is not possible (e.g. override file config with command arguments). This would ease sharing the traefik.yml without exposing secrets like the pilot token.

pilot:
  token: "$TRAEFIK_PILOT_TOKEN"

see https://github.com/rsteube/paerser/commit/ca41a5f0c7d7dbd070e8e7c9b398a5dce3076844