upload/koji: reorganize run-koji-container script

run-koji-container has now two actions: start and stop:
- ./run-koji-container.sh start
- ./run-koji-container.sh stop

The start action starts all containers. When it exits, all containers are
started and running in the background. To stop and removethem, use the stop
action.

This change is needed so we're able to easily use this script also in the CI
environment.

internal/upload/koji/README.md

@@ -3,7 +3,7 @@
 Firstly, you need to start the koji container:
 
 ```
-sudo ./internal/upload/koji/run-koji-container.sh
+sudo ./internal/upload/koji/run-koji-container.sh start
 ```
 
 This command starts a kojihub instance available at
@@ -20,3 +20,9 @@ go test -v -tags koji_test ./internal/upload/koji
 
 The test is run on each PR in the Github CI. See `.github/workflows/tests.yml`
 for more details.
+
+To stop and remove the koji container, use the following command:
+
+```
+sudo ./internal/upload/koji/run-koji-container.sh stop
+```