Simplify docker usage, HOSTNAME variable is taken from container hostname

Bug: https://github.com/happyDomain/happydeliver/issues/3

docker/README.md

@@ -109,12 +109,13 @@ Default configuration for the Docker environment:
 
 The container accepts these environment variables:
 
-- `DOMAIN`: Email domain for test addresses (default: happydeliver.local)
-- `HOSTNAME`: Container hostname (default: mail.happydeliver.local)
+- `HAPPYDELIVER_DOMAIN`: Email domain for test addresses (default: happydeliver.local)
+
+Note that the hostname of the container is used to filter the authentication tests results.
 
 Example:
 ```bash
-docker run -e DOMAIN=example.com -e HOSTNAME=mail.example.com ...
+docker run -e HAPPYDELIVER_DOMAIN=example.com --hostname mail.example.com ...
 ```
 
 ## Volumes

docker/entrypoint.sh

@@ -4,7 +4,7 @@ set -e
 echo "Starting happyDeliver container..."
 
 # Get environment variables with defaults
-HOSTNAME="${HOSTNAME:-mail.happydeliver.local}"
+[ -n "${HOSTNAME}" ] || HOSTNAME=$(hostname)
 HAPPYDELIVER_DOMAIN="${HAPPYDELIVER_DOMAIN:-happydeliver.local}"
 
 echo "Hostname: $HOSTNAME"