text: Title: Unable to change nginx.conf in the image
  Body: This may seem a little strange, but I am not able to change nginx.conf file inside /etc/nginx/conf.d/nginx.conf
  
  Here is what I did:
  
  ## Method1: Change in Dockerfile
  My Dockerfile looks like this:
  ```
  FROM tiangolo/uwsgi-nginx-flask:flask
  COPY ./app /app
  COPY ./changes/nginx.conf /etc/nginx/conf.d/nginx.conf
  COPY ./changes/nginx.conf /app/
  ```
  
  ./changes/nginx.conf looks like this:
  ```
  server {
      location /app1/ {
          try_files $uri @app;
      }
      location @app {
          include uwsgi_params;
          uwsgi_pass unix:///tmp/uwsgi.sock;
      }
      location /static {
          alias /app/static;
      }
  }
  ```
  **Note the change in location in above server block from `location /` to `location /app1/`**
  
  After the image is built and I run the docker container, I exec into the running container
  `sudo docker exec -ti CONTAINER_ID /bin/bash`
  
  `cat /app/nginx.conf` shows presence of updated nginx.conf file (location changes from `/` to `/app1/`
  
  BUT `cat /etc/nginx/conf.d/nginx.conf` still shows the old conf file (location is still `/`)
  I thought maybe the second COPY line is not getting executed successfully and docker isn't throwing error on console (sudo?). So, I changed the conf file manually and did a docker commit -  the second approach mentioned below.
  
  ## Method2: Docker commit
  
  After the docker container was up and running, I used exec to login into the container using 
  `[vagrant@localhost]$ sudo docker exec -ti CONTAINER_ID /bin/bash`
  
  `[root@CONTAINER_ID]# vi /etc/nginx/conf.d/nginx.conf`
  Changing the file to reflect below:
  ```
  server {
      location /app1/ {
          try_files $uri @app;
      }
      location @app {
          include uwsgi_params;
          uwsgi_pass unix:///tmp/uwsgi.sock;
      }
      location /static {
          alias /app/static;
      }
  }
  ```
  Saved the file `wq!` and exit the container.
  After that I did `sudo docker commit CONTAINER_ID my_new_image`
  
  Starting a new container and re-logging into container running on my_new_image still gives below nginx.conf file inside /etc/nginx/conf.d/nginx.conf:
  ```
  server {
      location / {
          try_files $uri @app;
      }
      location @app {
          include uwsgi_params;
          uwsgi_pass unix:///tmp/uwsgi.sock;
      }
      location /static {
          alias /app/static;
      }
  }
  ```
  I can tell that the my_new_image has some changes because it is larger in size than tiangolo/uwsgi-nginx-flask-docker because I had installed vim to edit the file. But somehow file changes are not persisting inside /etc/nginx/conf.d/nginx.conf.
  
  Am I doing something wrong or is it some bug?
  