Allow docker secret files for sensitive environment variables

[freakingawesome]

This allows you to specify the location of a file to make it easier to use with Docker secrets. If you specify the environment variable directly, it won't try to read from a file.

For example:

portus:
    environment:
        ...
        PORTUS_PRODUCTION_PASSWORD_FILE: /run/secrets/portus-db-root-password
        PORTUS_SECRET_KEY_BASE_FILE: /run/secrets/portus-secret-key-base
        PORTUS_PASSWORD_FILE: /run/secrets/portus-password
        ...

[Nols1000]

Why has this pull request not yet been merged?

[flavio]

@mssola can you take a look at that please?

[Vad1mo]

I don't think this is the right place. It would be easy for Portus to Support docker secretes/files out of the box. Could you open an issue? We will implement it as its also needed for us.

[potzkovge]

i made an alternative solution which represents more the docker standard used in other repositories #99