Change additional cert file name to avoid conflicts

What does this MR do?

Update kaniko documentation for add custom CA certs to a file that will not conflict with public certs embedded in the kaniko container.

When a file '/kaniko/ssl/certs/ca-certificates.crt' is present with valid cert content, pulling containers from public repos will fail with "x509: certificate signed by unknown authority", however pulling / pushing from / to custom CA endpoints will be successful.

The workaround is to change the name of the custom CA cert bundle added to the kaniko's '/kaniko/ssl/certs/' path. In my case I called it 'additional-ca-cert-bundle.crt' and used the same content that is passed to GItLab security containers to perform scans.
The following is the full line of code used to make it work in my custom cert environment:

mkdir -p /kaniko/ssl/certs/  &&  touch /kaniko/ssl/certs/ca-certificates.crt  && touch /kaniko/ssl/certs/additional-ca-cert-bundle.crt &&  echo "$ADDITIONAL_CA_CERT_BUNDLE" > /kaniko/ssl/certs/additional-ca-cert-bundle.crt

This was not included in the documentation change, as it was not tested in enough target environments to validate that this level of complexity is required in all environments.

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