fix: Copy bins and keys with mounts for ostree commit (#132)

I've been investigating more into how ostree works and how it relates to
running `ostree container commit` for each layer. I've decided to move
our pre-installed bins and public keys into their own stages and then
bind mount them into a `RUN` instruction so that we can just use `cp` to
get the files into the image and then call `ostree container commit`.
Now all of our layers in the image (after the base image) will be in the
ostree commit tree.

integration-tests/test-repo/config/recipe.yml

@@ -33,8 +33,7 @@ modules:
       remove:
         - org.gnome.eog
 
-  # Needs a bug to be fixed to allow / in image name
-  # - type: signing
+  - type: signing
 
   - type: test-module