ci: restrict branch names for build-containers workflow (star-bnl#127)

This prevents the workflow from failing on pr/ branches pushed
directly into the main repo. Feature branches could temporarily relax
this constraint as needed.

Alternatively we could set `on.push.branches-ignore` to `'pr/*'`.

(cherry picked from commit 5d41b66)

.github/workflows/build-containers.yml

@@ -1,6 +1,12 @@
 name: Build Containers
 
-on: [push]
+on:
+  push:
+    branches:
+      - main
+      - 'SL*'
+    tags:
+      - '*'
 
 jobs:
   ROOT5: