[Automation] Testing against internal Couchbase Server builds

Now that we have some automation in place, it would be good to identify how we can test our code samples/n1ql queries against internal builds of Couchbase Server.

Currently our automation tests run the risk of failing more regularly if this is not addressed, we would need to be able to access docker builds such as this:`build-docker.couchbase.com:443/couchbase/server-internal:7.0.0-4960` possibly with some doc-team credentials.

UPDATE:
I've setup an account for docs in Gitlab, thanks to help from Christopher Hillery. We should be able to use the account I created (`docs-docker-bot`) to pull latest docker builds tested by QE(which are usually available on a Friday) here: registry.gitlab.com/cb-vanilla/server:7.0.0-5017

We can use Github Actions secrets to ensure we have some level of security for storing credentials:
https://docs.github.com/en/actions/reference/encrypted-secrets

Gitlab allows us to create a personal access token, so we should use that to pull the image from the private docker registry.