Added configureable base image to minecraft containers

minecraft/forge/.env.example

@@ -1,3 +1,8 @@
+# This setting overwrites the base image from which the final image is build.
+# Unless you have a reason to change this (e.g. using an ARM version), you can
+# just leave this blank.
+BASE_IMAGE=
+
 # Build arguments
 MC_VERSION=
 FORGE_VERSION=
@@ -9,8 +14,8 @@ XMX=4
 
 # Mount points
 # The mods should be placed inside the CONFIG_DIR under `mods`
-CONFIG_DIR=
-WORLDS_DIR=
+CONFIG_DIR=config
+WORLDS_DIR=worlds
 
 # Other
 PORT=25565