manifest/RawOSTreeImage: don't hardcode FS root mount name

The `RawOSTreeImage` `serialize()` method made an assumption when
generating the org.osbuild.copy stage options, that the mount name
for the filesystem root is "root". While this worked so far, the
method should not rely on this behavior, since it would break if
we change the implementation of escaping mountpoint paths.

Modify the function to determine the mount name of the Entity
based on the mountpoint path while iterating over all partition table
entities. Specifically, use the mount name of the one with mountpoint
`/`.

Signed-off-by: Tomáš Hozza <[email protected]>

pkg/manifest/raw_ostree.go

@@ -83,10 +83,24 @@ func (p *RawOSTreeImage) serialize() osbuild.Pipeline {
 			"ostree-tree": *osbuild.NewOSTreeCheckoutInput("org.osbuild.source", commitChecksum),
 		}
 
+		// Find the FS root mount name to use as the destination root
+		// for the target when copying the boot files.
+		var fsRootMntName string
+		for _, mnt := range *bootCopyMounts {
+			if mnt.Target == "/" {
+				fsRootMntName = mnt.Name
+				break
+			}
+		}
+
+		if fsRootMntName == "" {
+			panic("no mount found for the filesystem root")
+		}
+
 		for _, paths := range bootFiles {
 			bootCopyOptions.Paths = append(bootCopyOptions.Paths, osbuild.CopyStagePath{
 				From: fmt.Sprintf("input://ostree-tree/%s%s", commitChecksum, paths[0]),
-				To:   fmt.Sprintf("mount://root%s", paths[1]),
+				To:   fmt.Sprintf("mount://%s%s", fsRootMntName, paths[1]),
 			})
 		}