Predefined CI variables for the dependency proxy

🔍 What does this MR do?

This adds the following predefined environment variables for use with the dependency proxy:

• CI_DEPENDENCY_PROXY_SERVER
• CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX
• CI_DEPENDENCY_PROXY_USER
• CI_DEPENDENCY_PROXY_PASSWORD

This will allow users to generalize commands similar to:

docker login -u username -p password gitlab.com
docker pull gitlab.com/my-group/dependency_proxy/containers/alpine:latest

in CI scripts with variables:

docker login -u "$CI_DEPENDENCY_PROXY_USER" -p "$CI_DEPENDENCY_PROXY_PASSWORD" "$CI_DEPENDENCY_PROXY_SERVER"
docker pull "$CI_DEPENDENCY_PROXY_GROUP_IMAGE_PREFIX"/alpine:latest

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #280582 (closed)