buildbuddy.yaml: Use container_image: "ubuntu-20.04".

buildbuddy.yaml

@@ -1,6 +1,7 @@
 # "This config is equivalent to the default config that we use if you do not have a buildbuddy.yaml file at the root of your repo."
 actions:
   - name: "Test all targets"
+    container_image: "ubuntu-20.04" # <-- we use .deb binaries from 20.04 as our dependencies
     triggers:
       push:
         branches:
@@ -13,6 +14,7 @@ actions:
       - "test --config=remote //:util_test" # <-- main modification here: --config=remote as a default for buildbuddy.yaml; also currently testing even //:all is broken (because it tries to compile macclipboard.m), much less //...
 
   - name: "Build main binary"
+    container_image: "ubuntu-20.04" # <-- we use .deb binaries from 20.04 as our dependencies
     triggers:
       push:
         branches: