pinned ubuntu version to 20.04 (#746)

Author: janniclas

.github/workflows/docker.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   push_to_registries:
     name: Push Docker image to multiple registries
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     permissions:
       packages: write
       contents: read