Make sure gunicorn runs inside $DATA_DIR path

- Changes directory to $DATA_DIR to execute the gunicorn command in the
docker entry cmd script. This ensures that satosa will find
proxy_conf.yaml and the relevant files (plugins etc)
- Solves issue where docker image was broken after #278

Author: ioparaskev

docker/start.sh

@@ -57,9 +57,9 @@ then chain_opts="--ca-certs chain.pem"
 fi
 
 # start the proxy
-exec gunicorn $conf_opt        \
+(cd $DATA_DIR; exec gunicorn $conf_opt        \
 	-b 0.0.0.0:"${PROXY_PORT}" \
 	satosa.wsgi:app            \
 	$https_opts                \
 	$chain_opts                \
-	;
+	;)