podman: don't affect --shell stdout with podman image umout

The `podman image umount` actually dumps the image ID to stdout.

Complements: b626045
Closes: #1218

mock/py/mockbuild/podman.py

@@ -101,7 +101,7 @@ def mounted_image(self):
         finally:
             logger.info("umounting image %s (%s) with podman image umount",
                         self.image, mountpoint)
-            subprocess.run(cmd_umount, check=True)
+            subprocess.run(cmd_umount, capture_output=True, check=True)
 
     @traceLog()
     def cp(self, destination, tar_cmd):

releng/release-notes-next/podman-image-unmount.bugfix

@@ -0,0 +1,2 @@
+The `--shell` standard output is no longer affected by `podman image unmount`
+output executed in the background (prints out the image ID).