Introduce multistage Docker build for optimized image size

Implemented a multistage Docker build, separating build and runtime stages to reduce the final image size and improve security. Adjusted `.dockerignore` to exclude unnecessary files from the build context and updated the base JRE image to use the build output efficiently.

.dockerignore

@@ -1,15 +1,8 @@
 .git
 .github
 .idea
-.mvn
-src
 classes
 logs
-target/*
-!target/netneutrality.jar
+target
 *.iml
 README.md
-checkstyle.xml
-mvnw
-mvnw.cmd
-pom.xml

Dockerfile

@@ -1,3 +1,14 @@
+FROM eclipse-temurin:25-jdk AS build
+
+WORKDIR /build
+
+COPY .mvn .mvn
+COPY mvnw pom.xml checkstyle.xml ./
+RUN chmod +x mvnw && ./mvnw -B -q dependency:go-offline
+
+COPY src src
+RUN ./mvnw -B -q package -DskipTests
+
 FROM eclipse-temurin:25-jre@sha256:ff82c74a630c08b417502c4c18189e62074edbb28eb1f2d31b799ac138c01058
 
 RUN apt-get update && apt-get install -y --no-install-recommends netcat-openbsd curl && rm -rf /var/lib/apt/lists/*
@@ -8,7 +19,7 @@ ARG API_PORT=9000
 EXPOSE ${API_PORT}
 
 COPY startup.sh ./
-COPY target/netneutrality.jar ./
+COPY --from=build /build/target/netneutrality.jar ./
 
 RUN ["chmod", "+x", "./startup.sh"]
-CMD ["./startup.sh"]
+CMD ["./startup.sh"]