fedora-common-ostree: Default to bash

That said, maybe someone wants to do something else for these
images, but this is kind of the lowest common denominator...

Part of https://pagure.io/releng/issue/11324

fedora-common-ostree.yaml

@@ -3,6 +3,10 @@ ref: fedora/rawhide/${basearch}/ostree-base
 automatic_version_prefix: "39"
 mutate-os-release: "39"
 
+# Default to `bash` in our container, the same as other containers we ship.
+container-cmd:
+  - /usr/bin/bash
+
 include: fedora-common-ostree-pkgs.yaml
 
 # See https://github.com/coreos/bootupd