Use dependency proxy to pull images from DockerHub

This should improve latency and throughput

.gitlab/ci/cng.gitlab-ci.yml

 cloud-native-image:
   extends: .cng:rules
-  image: ruby:2.7-alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7-alpine
   dependencies: []
   stage: post-test
   variables:

.gitlab/ci/docs.gitlab-ci.yml

@@ -2,7 +2,7 @@
   extends:
     - .default-retry
     - .docs:rules:review-docs
-  image: ruby:2.7-alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7-alpine
   stage: review
   needs: []
   variables:

.gitlab/ci/frontend.gitlab-ci.yml

@@ -259,13 +259,13 @@ coverage-frontend:
 
 qa-frontend-node:10:
   extends: .qa-frontend-node
-  image: node:dubnium
+  image: ${GITLAB_DEPENDENCY_PROXY}node:dubnium
 
 qa-frontend-node:latest:
   extends:
     - .qa-frontend-node
     - .frontend:rules:qa-frontend-node-latest
-  image: node:latest
+  image: ${GITLAB_DEPENDENCY_PROXY}node:latest
 
 webpack-dev-server:
   extends:

.gitlab/ci/global.gitlab-ci.yml

@@ -132,7 +132,7 @@
     FOSS_ONLY: '1'
 
 .use-docker-in-docker:
-  image: docker:${DOCKER_VERSION}
+  image: ${GITLAB_DEPENDENCY_PROXY}docker:${DOCKER_VERSION}
   services:
     - docker:${DOCKER_VERSION}-dind
   variables:

.gitlab/ci/notify.gitlab-ci.yml

 .notify-slack:
-  image: alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}alpine
   stage: notify
   dependencies: []
   cache: {}

.gitlab/ci/qa.gitlab-ci.yml

@@ -47,7 +47,7 @@ update-qa-cache:
     policy: push  # We want to rebuild the cache from scratch to ensure stale dependencies are cleaned up.
 
 .package-and-qa-base:
-  image: ruby:2.7-alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7-alpine
   stage: qa
   retry: 0
   script:

.gitlab/ci/releases.gitlab-ci.yml

@@ -4,7 +4,7 @@
 .merge-train-sync:
   # We don't need/want any global before/after commands, so we overwrite these
   # settings.
-  image: alpine:edge
+  image: ${GITLAB_DEPENDENCY_PROXY}alpine:edge
   stage: sync
   before_script:
     - apk add --no-cache --update curl bash jq

.gitlab/ci/review.gitlab-ci.yml

@@ -25,7 +25,7 @@ review-build-cng:
   extends:
     - .default-retry
     - .review:rules:review-build-cng
-  image: ruby:2.7-alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7-alpine
   stage: review-prepare
   before_script:
     - source ./scripts/utils.sh
@@ -199,7 +199,7 @@ review-performance:
 parallel-spec-reports:
   extends:
     - .review:rules:mr-only-manual
-  image: ruby:2.7-alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7-alpine
   stage: post-qa
   dependencies: ["review-qa-all"]
   variables:

.gitlab/ci/setup.gitlab-ci.yml

@@ -26,7 +26,7 @@ cache gems:
 dont-interrupt-me:
   extends: .setup:rules:dont-interrupt-me
   stage: sync
-  image: alpine:edge
+  image: ${GITLAB_DEPENDENCY_PROXY}alpine:edge
   interruptible: false
   variables:
     GIT_STRATEGY: none
@@ -52,7 +52,7 @@ no_ee_check:
 verify-tests-yml:
   extends:
     - .setup:rules:verify-tests-yml
-  image: ruby:2.7-alpine
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7-alpine
   stage: test
   needs: []
   script:
@@ -61,7 +61,7 @@ verify-tests-yml:
     - scripts/verify-tff-mapping
 
 .detect-test-base:
-  image: ruby:2.7
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7
   needs: []
   stage: prepare
   script:

.gitlab/ci/test-metadata.gitlab-ci.yml

 .tests-metadata-state:
-  image: ruby:2.7
+  image: ${GITLAB_DEPENDENCY_PROXY}ruby:2.7
   before_script:
     - source scripts/utils.sh
   artifacts:

.gitlab/ci/workhorse.gitlab-ci.yml

 workhorse:
   extends: .workhorse:rules:workhorse
-  image: golang:1.14
+  image: ${GITLAB_DEPENDENCY_PROXY}golang:1.14
   stage: test
   needs: []
   script: