Rebase to ublue base

Primary motivation here is that ublue base, as compared to vanilla
Silverblue, provides RPMFusion packages.

This is really important to us because before this patch, OBS streaming
errors out with "Failed to open OpenH264: Encoder not found".

Containerfile

@@ -15,7 +15,7 @@
 # - "base"
 #
 #  "aurora", "bazzite", "bluefin" or "ucore" may also be used but have different suffixes.
-ARG SOURCE_IMAGE="silverblue"
+ARG SOURCE_IMAGE="base"
 
 ## SOURCE_SUFFIX arg should include a hyphen and the appropriate suffix name
 # These examples all work for silverblue/kinoite/sericea/onyx/lazurite/vauxite/base