BE: repair Dockerfile + docs

flask_be/Dockerfile

@@ -4,6 +4,6 @@ COPY . /usr/local/app/
 RUN ["pip", "install", "waitress"]
 WORKDIR /usr/local/app/
 RUN ["pip", "install", "-r", "requirements.txt"]
-CMD ["waitress-serve", "--listen=127.0.0.42:5000", "app:app"]
+CMD ["waitress-serve", "--listen=0.0.0.0:5000", "app:app"]
 
 EXPOSE 5000

server_readme.md

@@ -30,6 +30,7 @@ PYTHON:
 
 DOCKER:
 - docker exec -it CONTAINER bash  -->  explore filesystem Docker of specified *container* (with bash); more generally, 'docker exec' will execute commands on it. The container has to be running.
+- docker [buildx] build -t IMAGE_NAME .  -->  build docker image of specified name using a Dockerfile in the current working directory. The buildx extra keyword is needed on Windows.
 - docker run -d --name CONTAINER -p 80:80 IMAGE  -->  example of running Docker: run specified *image* in *container* named 'CONTAINER'; expose internal port 80 on external port 80 (-p); detach (-d)
 - docker images  -->  check existing Docker images
 - docker ps [-a]  -->  check running [all] docker containers