Refactor Drone CI and Dockerfile for optimized builds and runtime

Streamlined `.drone.yml` by removing redundant test-and-jar step, introducing image caching with `cache_from`, and refining conditional triggers. Enhanced `Dockerfile` with multi-stage builds for dependency warming, test execution, and efficient package creation, alongside runtime optimizations for graceful shutdown and resource management.

Dockerfile

@@ -1,14 +1,50 @@
-# ---- build ----
-FROM gradle:8.10.2-jdk21 AS build
+# ---------- Base build image (pins Gradle + JDK) ----------
+FROM gradle:8.10.2-jdk21-alpine AS build-base
 WORKDIR /workspace
-COPY . .
-RUN gradle clean bootJar --no-daemon
 
-# ---- runtime ----
-FROM eclipse-temurin:21-jre
-ENV JAVA_OPTS="-XX:+UseContainerSupport -XX:MaxRAMPercentage=75"
+# Pre-copy only files that affect dependency resolution
+COPY settings.gradle.kts settings.gradle.kts
+COPY settings.gradle settings.gradle
+COPY build.gradle.kts build.gradle.kts
+COPY build.gradle build.gradle
+COPY gradle gradle
+COPY gradlew gradlew
+
+# Warm Gradle wrapper & dependency cache (layer-cached in the image)
+RUN ./gradlew --version || gradle --version
+RUN ./gradlew --no-daemon --stacktrace dependencies || true
+
+# ---------- Test stage (fail fast but reuse the warmed deps) ----------
+FROM build-base AS test
+# Copy the full project now (invalidates later layers only when code changes)
+COPY . .
+RUN ./gradlew --no-daemon clean test
+
+# ---------- Package stage (reuses compiled classes incrementally) ----------
+FROM test AS package
+RUN ./gradlew --no-daemon bootJar
+# Find the built jar path
+RUN ls -lah build/libs
+
+# ---------- Runtime image ----------
+FROM eclipse-temurin:21-jre-alpine
+RUN addgroup -S app && adduser -S app -G app
+USER app
 WORKDIR /app
-COPY --from=build /workspace/build/libs/*-SNAPSHOT.jar app.jar
-EXPOSE 8080
+
+# Copy the application jar
+COPY --from=package /workspace/build/libs/*SNAPSHOT*.jar /app/app.jar
+
+# Healthcheck via Spring Actuator (optional if you have it)
+# HEALTHCHECK --interval=30s --timeout=3s --start-period=20s --retries=3 \
+#   CMD wget -qO- http://localhost:8080/actuator/health/liveness | grep -q '"status":"UP"' || exit 1
+
+# Graceful shutdown
+ENV JAVA_OPTS="-XX:+UseContainerSupport -XX:MaxRAMPercentage=75 -XX:InitialRAMPercentage=50"
+ENV SERVER_SHUTDOWN=graceful
+ENV SPRING_LIFECYCLE_TIMEOUT=20s
+
 LABEL com.centurylinklabs.watchtower.enable=true
-ENTRYPOINT ["sh","-c","java $JAVA_OPTS -jar /app/app.jar"]
+
+EXPOSE 8080
+ENTRYPOINT ["sh","-c","java $JAVA_OPTS -Dserver.shutdown=${SERVER_SHUTDOWN} -Dspring.lifecycle.timeout-per-shutdown-phase=${SPRING_LIFECYCLE_TIMEOUT} -jar /app/app.jar"]