Add a mechanism to override all of the relay's env variables without explicitly exposing them manually

[natanasow]

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👾 Description of the issue

Currently, there is no natural way to override a relay's env if it's been added recently and not manually exposed in the local node as a config variable. All of the available overrides are described here and they are mapped to the relay's container here, again manually.

Proposed Solution:

Аdd an ability to pass an entire user's defined .env file (or a single env variable) to the relay's container.

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[hendrikebbers]

@natanasow, can you add some more detail to the description? For me, it is not really clear what needs to be done.