feat(minecraft-server): replace environment variables in configs

Add additional setup script that replaces environment variables inside
config files.

Closes #298

minecraft-server/Dockerfile

@@ -3,19 +3,19 @@ FROM openjdk:8u171-jre-alpine
 LABEL maintainer "itzg"
 
 RUN apk add --no-cache -U \
-          openssl \
-          imagemagick \
-          lsof \
-          su-exec \
-          shadow \
-          bash \
-          curl iputils wget \
-          git \
-          jq \
-          mysql-client \
-          tzdata \
-          rsync \
-          python python-dev py2-pip
+  openssl \
+  imagemagick \
+  lsof \
+  su-exec \
+  shadow \
+  bash \
+  curl iputils wget \
+  git \
+  jq \
+  mysql-client \
+  tzdata \
+  rsync \
+  python python-dev py2-pip
 
 RUN pip install mcstatus
 
@@ -71,7 +71,8 @@ ENV UID=1000 GID=1000 \
   JVM_XX_OPTS="-XX:+UseG1GC" MEMORY="1G" \
   TYPE=VANILLA VERSION=LATEST FORGEVERSION=RECOMMENDED SPONGEBRANCH=STABLE SPONGEVERSION= LEVEL=world \
   PVP=true DIFFICULTY=easy ENABLE_RCON=true RCON_PORT=25575 RCON_PASSWORD=minecraft \
-  LEVEL_TYPE=DEFAULT GENERATOR_SETTINGS= WORLD= MODPACK= MODS= SERVER_PORT=25565 ONLINE_MODE=TRUE CONSOLE=true SERVER_NAME="Dedicated Server"
+  LEVEL_TYPE=DEFAULT GENERATOR_SETTINGS= WORLD= MODPACK= MODS= SERVER_PORT=25565 ONLINE_MODE=TRUE CONSOLE=true SERVER_NAME="Dedicated Server" \
+  REPLACE_ENV_VARIABLES="FALSE" ENV_VARIABLE_PREFIX="CFG_"
 
 COPY start* /
 RUN dos2unix /start* && chmod +x /start*