Fix shell variable expansion in docker manifest printf

Use double quotes in printf format string so ${PROJECT_ID} and
${IMAGE_NAME} are expanded by the shell. Single quotes prevented
expansion after #66 replaced ${{ env.* }} with shell variables.

Author: ericmj

.github/workflows/main.yml

@@ -136,4 +136,4 @@ jobs:
         run: |
           docker buildx imagetools create \
             -t gcr.io/${PROJECT_ID}/${IMAGE_NAME}:${COMMIT_SHORT_SHA} \
-            $(printf 'gcr.io/${PROJECT_ID}/${IMAGE_NAME}@sha256:%s ' *)
+            $(printf "gcr.io/${PROJECT_ID}/${IMAGE_NAME}@sha256:%s " *)