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

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

Closes #298

minecraft-server/start-finalSetup05EnvVariables

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+if [ "$REPLACE_ENV_VARIABLES" = "TRUE" ]; then
+  echo "Replacing env variables in configs that match the prefix $ENV_VARIABLE_PREFIX..."
+  while IFS='=' read -r name value ; do
+    # check if name of env variable matches the prefix
+    # sanity check environment variables to avoid code injections
+    if [[ "$name" = $ENV_VARIABLE_PREFIX* ]] && [[ $value =~ ^[0-9a-zA-Z_\-:/=?.+]*$ ]] && [[ $name =~ ^[0-9a-zA-Z_\-]*$ ]]; then
+        echo "$name = $value"
+        find /data/ -type f -exec sed -i 's#${'"$name"'}#'"$value"'#g' {} \;
+    fi
+  done < <(env)
+fi
+
+exec /start-minecraftFinalSetup $@