Allow to override default env variables with .env file

[leonidboykov]

The godotenv.Load method does not overload the ENV variable if it is already present. This is a problem if you want to use --env-file to provide any settings with default values in Dockerfile. This means, that it's not possible to set DRONE_DATABASE_SECRET within .env file (trying to achieve the technique described here).

In the case of deploying the Drone, I'm sure that using godotenv.Overload is preferable, because this allows to keep the database password private with docker secrets.

---

My current workaround is using entrypoint to unset env variables:

entrypoint:
  - /bin/sh
  - -c
  - |
    set -e
    unset DRONE_DATABASE_DATASOURCE
    set -- /bin/drone-server "$$@"
    exec "$$@"
command:
  - /bin/drone-server
  - --env-file=/drone.secret.env

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