Fix berkeley image tag

Previously we were using  prefix + the branch name. Since the intent of
the workflow is to run on only a specific branch, that was redundant.

Author: naglepuff

.github/workflows/berkeley-image.yml

@@ -37,9 +37,8 @@ jobs:
             ghcr.io/microbiomedata/nmdc-server/${{ matrix.image }}
           flavor: |
             latest=false
-            prefix=berkeley
           tags: |
-            type=ref,event=branch
+            type=raw,value=berkeley
 
       - name: Build and push
         uses: docker/build-push-action@v5