ci: Allow to manually trigger the Docker build

This allows to manually trigger the Docker build also for other branches
than "main".

Signed-off-by: Martin Nonnenmacher <[email protected]>

Author: mnonnenmacher

.github/workflows/docker-build.yml

@@ -6,6 +6,7 @@ on:
       - main
     tags:
       - "[0-9]+.[0-9]+.[0-9]+"
+  workflow_dispatch:
 
 env:
   REGISTRY: ghcr.io