workflows: disable the daily build on forks (closes #1168)

We'll now need to define this repo variable on our clone in order to make this run.

.github/workflows/docker_latest.yml

@@ -46,6 +46,7 @@ jobs:
   push_to_registry:
     name: Push latest Docker image to Docker Hub
     runs-on: ubuntu-latest
+    if: ${{ vars.RUN_DAILY_BUILD }}
     strategy:
       matrix:
         python-version: [3.11]