Use the Dockerfile from the main branch

There is no way to change the Dockerfile in a tag. We need to
use the Dockerfile in the main branch. So even if we're buildin
the image for a stable version, we need to check out the main branch.

The Dockerfile will take care of checking out the proper tags.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

.github/workflows/build-and-push.yml

@@ -26,7 +26,6 @@ jobs:
         - name: "Checkout"
           uses: actions/checkout@v4
           with:
-            ref: ${{ inputs.ref }}
             path: src/github.com/cloudbase/garm
             fetch-depth: 0