Small fix in apt publisher.

apt-publisher/Makefile

 	# rsync to host
 	rsync --delete -raPv -e "$$SYNC_CMD_SSH" repo $$SYNC_CMD_DEST
 
-shell:
+shell-ubuntu1804:
 	docker pull rayburgemeestre/apt-ubuntu:18.04
 	docker run -i -t -v $$PWD:$$PWD -v $$PWD/gnupg:/root/.gnupg -v $$PWD/repo:/repo --workdir $$PWD rayburgemeestre/apt-ubuntu:18.04 /bin/bash
 
+shell-ubuntu2004:
+	docker pull rayburgemeestre/apt-ubuntu:20.04
+	docker run -i -t -v $$PWD:$$PWD -v $$PWD/gnupg:/root/.gnupg -v $$PWD/repo:/repo --workdir $$PWD rayburgemeestre/apt-ubuntu:20.04 /bin/bash
+
 clean-all:
 	rm -rf repo gnupg keys