inputs: pre-defined input paths

Instead of bind-mounting each individual input into the container,
create a temporary directory that is used by all inputs and bind-
mount this to the well known location ("/run/osbuild/inputs"). The
temporary directory is then passed to the input so that it can
make the requested resources available relative to that directory.
This is enforced by the common input handling code.
Additionally, pass the well known input path via a new "paths" key
to the arguments dictionary passed to the stage.

osbuild/inputs.py

@@ -25,6 +25,7 @@ import subprocess
 
 from typing import Dict, Optional, Tuple
 
+from osbuild.util.types import PathLike
 from .objectstore import StoreServer
 
 
@@ -58,13 +59,19 @@ class Input:
         m.update(json.dumps(self.options, sort_keys=True).encode())
         return m.hexdigest()
 
-    def run(self, storeapi: StoreServer) -> Tuple[str, Dict]:
+    def run(self, storeapi: StoreServer, root: PathLike) -> Tuple[str, Dict]:
         name = self.info.name
+
+        target = os.path.join(root, self.name)
+        os.makedirs(target)
+
         msg = {
             # mandatory bits
             "origin": self.origin,
             "refs": self.refs,
 
+            "target": target,
+
             # global options
             "options": self.options,
 
@@ -101,6 +108,11 @@ class Input:
         if r.returncode != 0:
             raise RuntimeError(f"{name}: error {r.returncode}")
 
-        path, data = reply["path"], reply.get("data", {})
+        path = reply["path"]
 
-        return path, data
+        if not path.startswith(root):
+            raise RuntimeError(f"returned {path} has wrong prefix")
+
+        reply["path"] = os.path.relpath(path, root)
+
+        return reply