Add the expose docker option (#9)

This example details how can socat container communicates with another
docker container that relies on docker exposed ports, like nginx-proxy
image.

README.md

@@ -57,6 +57,36 @@ $ docker run \
 * To automatically start the container on restart insert ```--restart always``` after ```docker run```.
 * To automatically start the container unless it has been stopped explicitly insert ```--restart unless-stopped``` after ```docker run```.
 
+## Use Case: Use nginx-proxy to access a local Cockpit instance
+
+Socat docker image by defintion does not use any EXPOSE inside Dockerfile. This may prejudice other containers that rely on this information, like nginx-proxy (https://github.com/nginx-proxy/nginx-proxy).
+
+Using expose will allow nginx-proxy to properly detect and communicate with socat instance without opening the port on host like ports option does.
+
+### Example
+In the following example, socat will be used to relay a host Cockpit instance to the nginx-proxy image, allowing to rely on proxy ports and optional Let's Encrypt support.
+
+```
+ cockpit-relay:
+ image: alpine/socat
+ container_name: cockpit-relay
+ depends_on:
+ - nginx-proxy
+ command: "TCP-LISTEN:9090,fork,reuseaddr TCP:172.17.0.1:9090"
+ expose:
+ - "9090"
+ environment:
+ - VIRTUAL_HOST=somehost.somedomain
+ - VIRTUAL_PROTO=https
+ - LETSENCRYPT_HOST=somehost.somedomain
+ - [email protected]
+ restart: unless-stopped
+ logging:
+ driver: journald
+ networks:
+ - webservices
+```
+
 # The Processes to build this image
 
 * Enable Travis CI cronjob on this repo to run build daily on master branch