feat: use environment variable for midnight container name management

[marco-martins]

I was following the docs to "Become a Midnight block producer" and noticed some issues in the steps for generating the Partner-chain keys.

The files midnight-shell.sh and midnight-node.sh use a hardcoded container name "midnight", but the compose.yml file uses a different name, "midnight-node".

As a result, when the container starts with Docker Compose, it uses the name "midnight-node", and the scripts midnight-shell.sh and midnight-node.sh are unable to find the container because they look for "midnight".

To fix this, I created a new environment variable to standardize the midnight node container name.

Hope this helps!

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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