stages/containers.storage: ability to specify a base file

In newer version of the container storage package the config file
moved from `/etc/containers` to `/usr/containers/`. The later is
not marked as config, so we don't want to change it.
The current containers code[1] will read _either_ a file in `usr`
or in `etc` depending on the existence of the latter. This we can
not just write the keys we want into a file in `/etc/containers`
without losing all other defaults set in the config file.
A new option `filebase` is therefore added, that when given will
be read and form the bases of the configuration data. Then data
from the target file (given via `filename`) will be merged into
and finally the actual configuration will be applied on top.

[1] 232bf398bd/types/options.go (L85)

stages/org.osbuild.containers.storage.conf

@@ -87,6 +87,10 @@ SCHEMA = r"""
       "/usr/share/containers/storage.conf"
     ]
   },
+  "filebase": {
+    "type": "string",
+    "description": "Read the base configuration from this file."
+  },
   "comment": {
     "type": "array",
     "items": {
@@ -166,14 +170,24 @@ def main(tree, options):
     location = options.get("filename", DEFAULT_LOCATION)
     config = options["config"]
     comment = options.get("comment", [])
+    filebase = options.get("filebase")
 
     path = os.path.join(tree, location.lstrip("/"))
     data = {}
 
-    with contextlib.suppress(FileNotFoundError):
-        with open(path, "r", encoding="utf8") as f:
+    # if a filebase was specified, we use it as base
+    if filebase:
+        with open(filebase, "r", encoding="utf8") as f:
             data = toml.load(f)
 
+    # if the target exists, we merge it
+    with contextlib.suppress(FileNotFoundError):
+        with open(path, "r", encoding="utf8") as f:
+            have = toml.load(f)
+
+            merge_config("storage", data, have)
+
+    # now merge our configuration into data
     merge_config("storage", data, config)
 
     with open(path, "w", encoding="utf8") as f: