Docker image deploy: set config vars (#21365)

* Add Hydro secrets as env vars

* Set app config-vars

* Add config vars previously used via app.json

.github/workflows/staging-deploy-pr-docker.yml

@@ -126,6 +126,8 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           HEROKU_API_TOKEN: ${{ secrets.HEROKU_API_TOKEN }}
+          HYDRO_ENDPOINT: ${{ secrets.HYDRO_ENDPOINT }}
+          HYDRO_SECRET: ${{ secrets.HYDRO_SECRET }
         with:
           script: |
             const esm = require('esm')