docs: document docker and make builds

README.md

@@ -8,6 +8,36 @@ Prometheus Spigot exposes server metrics at a configurable HTTP endpoint for Pro
 2. Copy the JAR from `build/libs/` into your server `plugins/` directory.
 3. Start the server once to generate `plugins/PrometheusSpigot/config.yml` and token.
 
+### Build with Docker
+
+If you do not have Gradle installed, you can build using Docker:
+
+```bash
+docker build -t prometheus-spigot-build .
+```
+
+Then extract the jar from the image:
+
+```bash
+docker create --name prometheus-spigot-build prometheus-spigot-build
+docker cp prometheus-spigot-build:/workspace/build/libs ./build/libs
+docker rm prometheus-spigot-build
+```
+
+### Build with Make
+
+If you prefer Make targets:
+
+```bash
+make build
+```
+
+Docker build + extract:
+
+```bash
+make docker-jar
+```
+
 ## Configure
 
 `config.yml` (defaults shown):