Introduce env example file

[Samk13]

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Description

build based on #304

Background

During review of the cookiecutter upgrade PR, we discussed reducing hard-coded CHANGE_ME values in docker-services and moving configuration into environment files.

The discussion surfaced a few possible approaches:

• Use a single .env file for both service/container configuration and Invenio application configuration.
• Keep service and application configuration separated (e.g. .containers-env and .invenio-env).
• Use an additional private configuration file such as invenio-private.cfg for application secrets, mounted at runtime rather than baked into images.
• Provide example configuration files (example.env or .env.example) and keep real environment files excluded from version control.

Goal

Evaluate whether the remaining hard-coded CHANGE_ME values should be moved to environment variables and whether we should consolidate configuration into a single .env-based workflow (or adopt another approach).

Checklist

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