ostree_container: make filename configurable, include arch

The default base name is probably fine in most cases, but there
are some where we might want to tweak it. We already allow this
for other phases (e.g. the livemedia phase).

Also, we should include the arch in the image filename. Not doing
this doesn't blow up the compose as, while they have identical
filenames, the images for different arches are in different paths,
but it's confusing for people who actually download and use the
images.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

pungi/checks.py

@@ -1102,6 +1102,7 @@ def make_schema():
                                     "$ref": "#/definitions/list_of_strings",
                                 },
                                 "subvariant": {"type": "string"},
+                                "name": {"type": "string"},
                             },
                             "required": [
                                 "treefile",