Require MINECRAFT_VERSION to be set manually, make no assumptions

Dockerfile

@@ -8,8 +8,6 @@
 
 FROM debian:jessie
 
-ENV MINECRAFT_VERSION 1.11.2
-
 RUN echo "deb http://overviewer.org/debian ./" >> /etc/apt/sources.list && \
     apt-get update && \
     apt-get install -y wget && \

entrypoint.sh

@@ -1,3 +1,11 @@
+#!/bin/bash
+set -o errexit
+
+if [ -z "$MINECRAFT_VERSION" ]; then
+  echo "Expecting environment variable MINECRAFT_VERSION to be set to non-empty string. Exiting."
+  exit 1
+fi
+
 wget https://s3.amazonaws.com/Minecraft.Download/versions/${MINECRAFT_VERSION}/${MINECRAFT_VERSION}.jar -P /home/minecraft/.minecraft/versions/${MINECRAFT_VERSION}/
 
 overviewer.py --config /home/minecraft/config.py