add .env variable USER_ID

To harmonize the configuration I added the possibility to change the
USER_ID variable in the .env file. Furthermore I appended the example
variable, incl. a explanatory comment, to the .env.sample.

.env.sample

@@ -1,2 +1,3 @@
+USER_ID=1000 # Set if your user id is different (`id -u`)
 APPLICATION_HOST=localhost
 ESDATA=../esdata

docker-compose.yml

@@ -5,7 +5,7 @@ services:
     build:
       context: .
       args: 
-        - USER_ID=1000
+        - USER_ID=${USER_ID=1000}
     volumes:
       - .:/app
     ports: