More flexible rpc secret file

[vic1707]

Breaking change

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Proposed change

As some IDEs and text editors like to leave an empty blank line at the end of a file you can end up with an empty RPC password when using docker secrets.
This change ensure we are taking the password from se second line of the secret file, no matter how many lines it contains.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to this container)
• Breaking change (fix/feature causing existing functionality to break)

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: relates to #
• Link to documentation updated (if done separately): https://...

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated

[silvanschermer]

i set up this image recently and I was a bit surprised that these are not set individually with the possibility of using _FILE declarations.