man: Add ostree-container-auth

Add a man page (which we'll figure out how to install later)
describing the state of things.

man/ostree-container-auth.md

@@ -0,0 +1,28 @@
+% ostree-container-auth 5
+
+# NAME
+ostree-container-auth description of the registry authentication file
+
+# DESCRIPTION
+
+The OSTree container stack uses the same file formats as **containers-auth(5)** but
+not the same locations.
+
+When running as uid 0 (root), the tooling uses `/etc/ostree/auth.json` first, then looks
+in `/run/ostree/auth.json`.  For any other uid, the file paths used are in `${XDG_RUNTIME_DIR}/ostree/auth.json`.
+
+In the future, it is likely that a path that is supported for both "system podman"
+usage and ostree will be added.
+
+## FORMAT
+
+The auth.json file stores, or references, credentials that allow the user to authenticate
+to container image registries.
+It is primarily managed by a `login` command from a container tool such as `podman login`,
+`buildah login`, or `skopeo login`.
+
+For more information, see **containers-auth(5)**.
+
+# SEE ALSO
+
+**containers-auth(5)**, **skopeo-login(1)**, **skopeo-logout(1)**