More simple nginx port mapping

Added the possibility to easily change the ports that nginx listens to.
This is useful, for instance when Apache or Nginx already runs on the
host. With this change, it is no longer necessary to edit the
docker-compose file to modify these ports.

To deploy with alternate ports, add a `.env` file with the following
contents:

```
OPENEDX_NGINX_PORT=8080
OPENEDX_NGINX_TLSPORT=8443
```

.gitignore

@@ -3,3 +3,4 @@ config/
 data-*/
 TODO
 openedx/requirements/private.*
+.env

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Changelog
 
+- 2018-11-17 [Improvement] Custom nginx port mapping. :crossed_swords: @frob @frohro
 - 2018-11-17 [Improvement] Add "make restart-openedx" command. :+1: @frob
 - 2018-11-13 [Improvement] Facilitate install of extra XBlocks. Thanks @frob!
 - 2018-10-30 [Bugfix] Fix rabbitmq restart policy

README.md

@@ -262,6 +262,28 @@ Additional requirements can be added to the `openedx/requirements/private.txt` f
     make build-openedx
     make run
 
+### "Cannot start service nginx: driver failed programming external connectivity"
+
+The containerized Nginx needs to listen to ports 80 and 443 on the host. If there is already a webserver, such as Apache or Nginx, running on the host, the nginx container will not be able to start. There are two solutions:
+
+1. Stop Apache or Nginx on the host:
+
+       sudo systemctl stop apache2
+       sudo systemctl stop nginx
+
+However, you might now want to do that if you need a webserver for running non-Open edX related applications. In such cases...
+
+2. Run the nginx container on different ports: you can create a `.env` file in the `openedx-docker` directory in which you indicate different ports. For instance:
+
+       cat .env
+       NGINX_HTTP_PORT=81
+       NGINX_HTTPS_PORT=444
+
+In this example, the nginx container ports would be mapped to 81 and 444, instead of 80 and 443.
+
+You should note that with the latter solution, it is your responsibility to configure the webserver on the host as a proxy to the nginx container. See [this](https://github.com/regisb/openedx-docker/issues/69#issuecomment-425916825) for http, and [this](https://github.com/regisb/openedx-docker/issues/90#issuecomment-437687294) for https.
+
+
 ### Help! Your containers are eating all my RAM/CPU/CHEESE
 
 You can identify which containers are consuming most resources by running:

docker-compose.yml

@@ -41,8 +41,8 @@ services:
     image: nginx:1.13
     restart: unless-stopped
     ports:
-      - "80:80"
-      - "443:443"
+      - "${NGINX_HTTP_PORT:-80}:80"
+      - "${NGINX_HTTPS_PORT:-443}:443"
     volumes:
       - ./config/nginx:/etc/nginx/conf.d/
       - ./data/lms:/openedx/data/lms:ro