Move container entrypoint to tier 0

tier-0/manifest.yaml

@@ -18,6 +18,10 @@ etc-group-members:
   - systemd-journal
   - adm
 
+# Default to `bash` in our container, the same as other containers we ship.
+container-cmd:
+  - /usr/bin/bash
+
 # Note that the default for c9s+ is sqlite; we can't rely on rpm being
 # in the target (it isn't in tier-0!) so turn this to host here.  This
 # does break the "hermetic build" aspect a bit.  Maybe eventually