Add extra step for composition of env variable

Composed environment variables cannot reference each other in the `env`-section. Therefore, use an extra step for composed env variables and write them to `$GITHUB_ENV`.

.github/workflows/main.yml

@@ -8,9 +8,6 @@ env:
   PROJECT_BASE_URL: https://qawareblog-2ixogl4y4q-ey.a.run.app/
   DOCKER_GAR_NAME: qawareblog-docker
   SERVICE_NAME: qawareblog
-  DOCKER_GAR_URL: "${{ env.RUN_REGION }}-docker.pkg.dev"
-  DOCKER_GAR_PATH: "${{ env.DOCKER_GAR_URL }}/{{ env.PROJECT_ID }}/{{ env.DOCKER_GAR_NAME}}"
-  DOCKER_IMAGE_PATH: "${{ env.DOCKER_GAR_PATH }}/{{ env.SERVICE_NAME }}:{{ github.sha }}"
 
 jobs:
   deploy:
@@ -21,6 +18,11 @@ jobs:
         with:
           submodules: true
 
+      - name: Setup composed environment variables
+        run: |
+          echo "DOCKER_IMAGE_PATH=${RUN_REGION}-docker.pkg.dev/${PROJECT_ID}/{$DOCKER_GAR_NAME}
+          /{$SERVICE_NAME}:${GITHUB_SHA}"  >> $GITHUB_ENV
+
       - name: Setup hugo
         uses: peaceiris/[email protected]
         with: