Fix ssh private key setup based on the public gitlab example

What does this MR do and why?

Based on the official gitlab ssh example the documentation looks outdated. I had lost quite a lot of time trying to setup the private key as file type variable but nothing worked. The ssh-private-key example comments do not specify that the variable needs to be a file type and it worked fine following the example. Also the ssh-add command is different. Setting up as file type the ssh-add "$SSH_PRIVATE_KEY" command fails with a libcrypto error.

Screenshots or screen recordings

None

How to set up and validate locally

Run a local docker ubuntu or alpine image and run the commands described in the original documentation here

MR acceptance checklist

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