build: add docker and make build tooling

.dockerignore

@@ -0,0 +1,6 @@
+build/
+.gradle/
+.idea/
+out/
+*.iml
+.DS_Store

Dockerfile

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+FROM gradle:8.6-jdk21
+
+WORKDIR /workspace
+
+COPY build.gradle.kts settings.gradle.kts /workspace/
+
+RUN gradle --no-daemon build -x test || true
+
+COPY src /workspace/src
+
+RUN gradle --no-daemon build

Makefile

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+IMAGE_NAME ?= prometheus-spigot-build
+CONTAINER_NAME ?= prometheus-spigot-build
+
+.PHONY: build docker-build docker-jar clean-docker
+
+build:
+	./gradlew build
+
+docker-build:
+	docker build -t $(IMAGE_NAME) .
+
+docker-jar: docker-build
+	docker create --name $(CONTAINER_NAME) $(IMAGE_NAME)
+	docker cp $(CONTAINER_NAME):/workspace/build/libs ./build/libs
+	docker rm $(CONTAINER_NAME)
+
+clean-docker:
+	docker rm -f $(CONTAINER_NAME) 2>/dev/null || true