build: push to Dockerhub and Quay

Dockerfile

-FROM        prom/busybox:latest
+FROM        quay.io/prometheus/busybox:latest
 MAINTAINER  The Prometheus Authors <prometheus-developers@googlegroups.com>
 
 COPY prometheus                             /bin/prometheus

circle.yml

 machine:
   environment:
     DOCKER_IMAGE_NAME: prom/prometheus
+    QUAY_IMAGE_NAME: quay.io/prometheus/prometheus
     DOCKER_TEST_IMAGE_NAME: quay.io/prometheus/golang-builder:1.5.4-main
     REPO_PATH: github.com/prometheus/prometheus
   pre:
@@ -21,8 +22,10 @@ dependencies:
     - |
       if [ -n "$CIRCLE_TAG" ]; then
         make docker DOCKER_IMAGE_NAME=$DOCKER_IMAGE_NAME DOCKER_IMAGE_TAG=$CIRCLE_TAG
+        make docker DOCKER_IMAGE_NAME=$QUAY_IMAGE_NAME DOCKER_IMAGE_TAG=$CIRCLE_TAG
       else
         make docker DOCKER_IMAGE_NAME=$DOCKER_IMAGE_NAME
+        make docker DOCKER_IMAGE_NAME=$QUAY_IMAGE_NAME
       fi
   post:
     - mkdir $CIRCLE_ARTIFACTS/binaries/ && cp -a .build/* $CIRCLE_ARTIFACTS/binaries/
@@ -38,7 +41,9 @@ deployment:
     owner: prometheus
     commands:
       - docker login -e $DOCKER_EMAIL -u $DOCKER_LOGIN -p $DOCKER_PASSWORD
+      - docker login -e $QUAY_EMAIL -u $QUAY_LOGIN -p $QUAY_PASSWORD quay.io
       - docker push $DOCKER_IMAGE_NAME
+      - docker push $QUAY_IMAGE_NAME
   hub_tag:
     tag: /^[0-9]+(\.[0-9]+){2}(-.+|[^-.]*)$/
     owner: prometheus
@@ -47,8 +52,11 @@ deployment:
       - promu release .tarballs
       - mkdir $CIRCLE_ARTIFACTS/releases/ && cp -a .tarballs/* $CIRCLE_ARTIFACTS/releases/
       - docker login -e $DOCKER_EMAIL -u $DOCKER_LOGIN -p $DOCKER_PASSWORD
+      - docker login -e $QUAY_EMAIL -u $QUAY_LOGIN -p $QUAY_PASSWORD quay.io
       - |
         if [[ "$CIRCLE_TAG" =~ ^[0-9]+(\.[0-9]+){2}$ ]]; then
             docker tag "$DOCKER_IMAGE_NAME:$CIRCLE_TAG" "$DOCKER_IMAGE_NAME:latest"
+            docker tag "$QUAY_IMAGE_NAME:$CIRCLE_TAG" "$QUAY_IMAGE_NAME:latest"
         fi
       - docker push $DOCKER_IMAGE_NAME
+      - docker push $QUAY_IMAGE_NAME