Run build on deprecated Ubuntu 20.04 in a container

Author: NWilson

.github/workflows/dev.yml

@@ -91,7 +91,8 @@ jobs:
   greatawk:
     # Tests with: GCC, -O3, oldest supported Ubuntu (in non-extended support)
     name: GCC -O3
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
+    container: ubuntu:20.04
     steps:
       - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
@@ -117,7 +118,8 @@ jobs:
   dodo:
     # Tests with: Autconf on oldest supported Ubuntu (in non-extended support)
     name: GCC -Os, old Autotools
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
+    container: ubuntu:20.04
     steps:
       - name: Checkout
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2