docs: Update podman build security options

It seems support was added into Buildah for no-new-privileges [1]
however the Podman build documentation was not updated.

Fixes #25731

[1] containers/buildah@d4c661a

Signed-off-by: Lewis Denny <[email protected]>

Author: ninja-quokka

docs/source/markdown/options/security-opt.image.md

@@ -16,7 +16,7 @@ container
 - `label=level:LEVEL`   : Set the label level for the container processes
 - `label=filetype:TYPE` : Set the label file type for the container files
 - `label=disable`       : Turn off label separation for the container
-- `no-new-privileges`   : Not supported
+- `no-new-privileges`   : Disable container processes from gaining additional privileges
 
 - `seccomp=unconfined` : Turn off seccomp confinement for the container
 - `seccomp=profile.json` :  JSON file to be used as the seccomp filter for the container.