Fix github Permission errors in Jenkins

Sometimes could happen that jenkins cannot work good with github repos in our project dependencies.

Errors could be something like:

git@github.com:zendframework/zend-uri.git
   Cloning into '/var/www/vendor/zendframework/zend-uri'...
   Host key verification failed.
   fatal: Could not read from remote repository.
   
   Please make sure you have the correct access rights
   and the repository exists.

or:

Cloning into 'zend-uri'...
   Permission denied (publickey).
   fatal: Could not read from remote repository.

For avoid these type of errors, you can add a public ssh key to your jenkins user:

1. First of all, you have to connect to the host and change the user to “jenkins”:

$ su jenkins

2. After that, try to see if we have already the ssh public key:

$ ssh -T git@github.com
Permission denied (publickey).

3. We don’t have it yet, so we create with (Hit enter at every request):

$ ssh-keygen -t rsa -b 4096 -C "your@githubaccount.email"
Generating public/private rsa key pair.
Enter file in which to save the key (/var/lib/jenkins/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /var/lib/jenkins/.ssh/id_rsa.
Your public key has been saved in /var/lib/jenkins/.ssh/id_rsa.pub.

4. Now we start the ssh agent:

$ eval "$(ssh-agent -s)"
Agent pid 1011

5. And add our new public key in the system:

$ ssh-add ~/.ssh/id_rsa
Identity added: /var/lib/jenkins/.ssh/id_rsa (/var/lib/jenkins/.ssh/id_rsa)

6. Ok, now we need to add this new public key to our account, follow this instructions.

7. Now we can try again to connect to github:

$ ssh -T git@github.com
Warning: Permanently added the RSA host key for IP address '192.30.253.113'
to the list of known hosts.
Hi github_user! You've successfully authenticated, but GitHub does not provide 
shell access.