workflows: build container test on Ubuntu 20.04

We need an older glibc for the binaries to work on older distros.

Author: bgilbert

.github/workflows/containers.yml

@@ -18,7 +18,7 @@ env:
 jobs:
   ditro-test:
     name: "Distro test"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         baseimage: ['debian:bullseye', 'ubuntu:20.04', 'ubuntu:22.04']