github: release docker container for next branch (#3086)

Build and release a docker container for the next branch.

Signed-off-by: David Bauer <mail@david-bauer.net>

.github/workflows/build-container.yml

@@ -5,6 +5,7 @@ on:
   push:
     branches:
       - 'master'
+      - 'next'
       - 'v202[0-9].[0-9].x'
     tags:
       - 'v*'