ci: Build the CI image

There currently is no build image in the flatpak org, so let's build
one!

In the future this should happen automatically because merge queues are
turned on and the CI actor isn't me anymore when merging.

Author: swick

.github/workflows/container.yml

@@ -1,5 +1,5 @@
 env:
-  IMAGE_TAG: 2025.03.26-6
+  IMAGE_TAG: 2025.08.12-0
 
 on:
   workflow_call: