stages: add new org.osbuild.bootc.install-to-filesystem

Support the `boot install to-filesystem` capability to install a
bootc image to a filesystem.

stages/org.osbuild.bootc.install-to-filesystem

@@ -0,0 +1,62 @@
+#!/usr/bin/python3
+"""Run bootc install to-filesystem
+
+Note that this needs the disk.img in the inputs as one continous
+devices so that bootupd can install grub to the mbr.
+
+Buildhost commands used: bootc
+"""
+
+import subprocess
+import sys
+
+import osbuild.api
+from osbuild.util import containers
+
+CAPABILITIES = ["CAP_MAC_ADMIN"]
+
+SCHEMA_2 = r"""
+"inputs": {
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["images"],
+  "properties": {
+    "images": {
+      "type": "object",
+      "additionalProperties": true
+    }
+  }
+},
+"options": {
+  "additionalProperties": false
+},
+"devices": {
+  "type": "object",
+  "additionalProperties": true
+},
+"mounts": {
+  "type": "array"
+}
+"""
+
+
+def main(inputs, mounts):
+    images = containers.parse_containers_input(inputs)
+    assert len(images) == 1
+    image = list(images.values())[0]
+
+    with containers.container_source(image) as (_, source):
+        dst = mounts["root"]["path"]
+        subprocess.run(
+            ["bootc", "install", "to-filesystem",
+             "--source-imgref", source,
+             "--skip-fetch-check", "--generic-image",
+             dst],
+            check=True
+        )
+
+
+if __name__ == "__main__":
+    args = osbuild.api.arguments()
+    r = main(args["inputs"], args["mounts"])
+    sys.exit(r)