Use CI_ACCESS_TOKEN instead of CI_REGISTRY_PASSWORD

What does this MR do?

Similar to codequality!46 (merged)

This MR updates our .gitlab-ci.yml to use $CI_ACCESS_TOKEN over $CI_REGISTRY_PASSWORD.

Why are we doing this?

For consistency purpose with codequality, we decided to now rely on $CI_ACCESS_TOKEN to prevent future upgrades to be broken when we need to release a new version.

1. Created an access token for the registry
2. Added it as a new CI variable
3. Updated our .gitlab-ci.yml
4. Tested it locally and it worked 🎉

docker login -u $CI_REGISTRY_USER -p $CI_ACCESS_TOKEN $CI_REGISTRY
WARNING! Using --password via the CLI is insecure. Use --password-stdin.
Login Succeeded

This MR changes how to login to the register by leveraging $CI_ACCESS_TOKEN which seems cleaner.