fix(ci): use correct Dockerfile

Author: caugner

.circleci/config.yml

@@ -34,7 +34,7 @@ jobs:
     steps:
       - checkout
       - run: make test-image IMAGE="php:fpm-alpine" DOCKERFILE="alpine"
-      - run: make test-image IMAGE="php:fpm-buster" DOCKERFILE="stretch"
+      - run: make test-image IMAGE="php:fpm-buster" DOCKERFILE="buster"
 
   test-7.1:
     machine: true
@@ -68,4 +68,4 @@ jobs:
       - checkout
       - run: make test-image IMAGE="php:7.4-fpm-alpine3.10" DOCKERFILE="alpine"
       - run: make test-image IMAGE="php:7.4-fpm-alpine3.11" DOCKERFILE="alpine"
-      - run: make test-image IMAGE="php:7.4-fpm-buster" DOCKERFILE="stretch"
+      - run: make test-image IMAGE="php:7.4-fpm-buster" DOCKERFILE="buster"