Update CI workflow to use 'main' branch

Author: adamjohnwright

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
             - synchronize
     push:
         branches:
-            - master
+            - main
 
 permissions:
     id-token: write
@@ -55,7 +55,7 @@ jobs:
                 path: /tmp/image.tar
 
     docker-push:
-        if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
+        if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/main' }}
         needs: docker-build
         runs-on: ubuntu-latest
         steps: