Problem while visualizing Robot in Gazebo: unable to prepare context: path "image/." not found

I'm following the Running a sample application with ROS Melodic and Gazebo 9 tutorial and I have made three directories, the first one, called HelloWorldSampleAppROSMelodicGazebo9 that contains only the Dockerfile for the base image, the second one, called HelloWorldSampleAppROSMelodicGazebo9RobotApp, that contains the Dockerfile for the robotic application and the robot-entrypoint.sh and the last one, HelloWorldSampleAppROSMelodicGazebo9SimApp, with the Dockerfile for the simapp and the simulation-entrypoint.sh file.

The point is that, when I run the following command:

$ docker build -t helloworldsampleapprosmelodicgazebo9robotapp:latest image/.

inside the folder for the robot image, I obtain the following error:

unable to prepare context: path "image/." not found

The same happens when I try to build the docker image for the simapp. What is the source of this error?