docker automated build was configured, adjust readme and makefile

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>

Makefile

@@ -58,5 +58,3 @@ push-files:
 docker:
 	if [ `systemctl is-active docker` = "inactive" ] ; then sudo systemctl start docker; fi
 	sudo docker build -t $(docker_repo)/univention-app-image:$(app_version) .
-	sudo docker login -u $(docker_login) -p $(docker_pwd)
-	sudo docker push $(docker_repo)/univention-app-image:$(app_version)

README.md

@@ -207,17 +207,21 @@ Uploads go against the current Nextcloud app version as configured in the Makefi
 
 ### Create and upload docker image
 
+to create a local build use
+
 $ make docker
 
-assumes that account credentials are stored in ``~/.docker-account-user`` and ``~/.docker-account-pwd``.
+in order to trigger a build on the docker hub tag it like
+
+$ git tag "13.0.3-0"
 
-The only downside is that it needs to ask for the password, because of using sudo for the docker commands. No, we do not encourage adding users to the docker group.
+and push.
 
 ### Prepare release
 
-$ make
+Tag the latest commit with the package name, e.g. `13.0.3-0` and push the tags. An automated build is configured at the docker hub.
 
-This is essentially ``make push-files && make docker``
+$ make push-files
 
 Afterwards, there are still two things that need to be done in the Provider Portal: