Added configureable base image to minecraft containers

minecraft/papermc/.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=
 PAPERMC_VERSION=

minecraft/papermc/Dockerfile

@@ -1,4 +1,8 @@
-FROM openjdk:11-jre-slim
+ARG BASE_IMAGE
+
+# We use ${:-} instead of a default value because the argument is always passed
+# to the build, it'll just be blank most likely
+FROM ${BASE_IMAGE:-'openjdk:11-jre-slim'}
 
 # Build arguments
 ARG MC_VERSION

minecraft/papermc/docker-compose.yml

@@ -4,6 +4,7 @@ services:
         build:
             context: .
             args:
+                - 'BASE_IMAGE'
                 - 'MC_VERSION'
                 - 'PAPERMC_VERSION'
         image: 'chewingbever/mc-papermc:${MC_VERSION}-${PAPERMC_VERSION}'