Support base containers without an EUPS tag in build-service.yml.

This gives us more flexibility in how we source the base container.
However, when there is no EUPS tag, the container labeling is much more
sensitive to how the workflow is started.

.github/workflows/build-service.yml

@@ -96,7 +96,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
       - name: Determine eups tag
         run: |
-          docker run ghcr.io/${{ github.repository_owner }}/prompt-base:"$BASE_TAG" bash -c "cat stack/miniconda*/ups_db/global.tags" > eups.tag
+          docker run ghcr.io/${{ github.repository_owner }}/prompt-base:"$BASE_TAG" bash -c "cat stack/miniconda*/ups_db/global.tags" > eups.tag || echo "Unknown" > eups.tag
           echo "Eups tag = $(< eups.tag)"
       - name: Build image
         run: |
@@ -122,7 +122,7 @@ jobs:
             docker tag $IMAGE_NAME $IMAGE_ID:$VERSION
             docker push $IMAGE_ID:$VERSION
             EUPS_TAG=$(< eups.tag)
-            if [ "$BASE_TAG" != "$EUPS_TAG" ]; then
+            if [ "$EUPS_TAG" != "Unknown" && "$BASE_TAG" != "$EUPS_TAG" ]; then
               if [ "$BRANCH" == "main" ]; then
                 VERSION="$EUPS_TAG"
               else