Issue #2241: add the dummy Dockerfile otobo.nginx-kerberos.dockerfile

Because Docker Hub checks the existence of the Dockerfile

otobo.nginx-kerberos.dockerfile

@@ -0,0 +1,17 @@
+# This Dockerfile is needed for the OTOBO nginx docker image with support for Kerberos.
+#
+# Actually, this file is not used for building the image. It is only a standin as
+# Docker Hub checks the existence of the configured Dockerfile when it automatically
+# builds images. For the actual build the hook in the script hooks/build is called.
+# That script declares that the Dockerfile otobo.nginx-kerberos.dockerfile is to be used for building the image.
+#
+# There is an extra build target otobo-ngix-kerberos that add support for Kerberos. This
+# complicated setup allows that a multiple images are build with a single Dockerfile. Thus
+# it is guaranteed that the images do not diverge.
+
+# See also bin/docker/build_docker_images.sh
+# See also https://docs.docker.com/docker-hub/builds/advanced/
+# See also https://doc.otobo.org/manual/installation/10.1/en/content/installation-docker.html
+
+# just to have a valid Dockerfile
+FROM nginx:mainline