Add build configuration system and paper image

- Add 'make paper' target to build a Paper image with plugins based on JRE
- Introduce dynamic .env and plugins.json configuration using Makefile
- Enable users to extend builds by managing their own directories in scratch/
- Implement copy_build_files macro for reproducible build management
- Add BUILDKIT_PROGRESS and DOCKER_BUILDKIT as configurable make vars

dockerfiles/Dockerfile.vanilla

@@ -8,7 +8,7 @@ USER minecraft
 WORKDIR /app
 
 # Download and verify sha1sum for server.jar
-RUN set -ux && \
+RUN set -eux && \
 # Grab latest version if not specified
   if [ "$VERSION" = "latest" ]; then \
     VERSION="$( \