lib/container_manifest: check for image before manifest

There's an asymmetry between `podman image exists` and `podman manifest
exists`. The first will return 1 if the image is a manifest. The second
will return 125 if the image is _not_ a manifest.

I think this is probably a bug in podman, but for now we can work around
this by reverting the order in which we do the checking. That way, if
the image exists but isn't a manifest, we'll return `True` and not
actually call `podman manifest exists` on it.

src/cosalib/container_manifest.py

@@ -26,8 +26,8 @@ def local_container_manifest_or_image_exists(repo, tag):
     @param repo str registry repository
     @param tag str tag
     '''
-    cmds = [["podman", "manifest", "exists", f"{repo}:{tag}"],
-            ["podman", "image", "exists", f"{repo}:{tag}"]]
+    cmds = [["podman", "image", "exists", f"{repo}:{tag}"],
+            ["podman", "manifest", "exists", f"{repo}:{tag}"]]
     for cmd in cmds:
         cp = runcmd(cmd, check=False)
         # The commands returns 0 (exists), 1 (doesn't exist), 125 (other error)