tests: Build the layering test images on scratch to avoid pulling swift:slim

Author: euanh

.github/workflows/interop_tests.yml

@@ -29,13 +29,17 @@ jobs:
             # First layer: payload does not have to be an executable, it just has to have known contents
             - name: Build first layer
               run: |
-                  echo first > payload
-                  swift run containertool --repository localhost:5000/layering_test payload
-                  docker run --pull=always --rm --entrypoint=cat localhost:5000/layering_test payload | grep first
+                  echo first layer > payload
+                  swift run containertool --repository localhost:5000/layering_test payload --from scratch
+                  docker create --name first --pull always localhost:5000/layering_test
+                  docker cp first:/payload first.payload
+                  grep first first.payload
 
             # Second layer: payload does not have to be an executable, it just has to have known contents.   It should replace the first layer.
             - name: Build another layer, which should override 'payload' from the first layer
               run: |
-                  echo second > payload
+                  echo second layer > payload
                   swift run containertool --repository localhost:5000/layering_test payload --from localhost:5000/layering_test:latest
-                  docker run --pull=always --rm --entrypoint=cat localhost:5000/layering_test payload | grep second
+                  docker create --name second --pull always localhost:5000/layering_test
+                  docker cp second:/payload second.payload
+                  grep second second.payload