ci: add Dockerfile and .dockerignore for containerisation

Dockerfile

@@ -0,0 +1,10 @@
+FROM amazoncorretto:21 AS builder
+WORKDIR /app
+ADD . .
+RUN chmod +x ./gradlew && \
+    ./gradlew build -x test
+
+FROM amazoncorretto:21-alpine AS runner
+WORKDIR /app
+COPY --from=builder /app/build/libs/delta-force-guide-server-1.0.0.jar app.jar
+ENTRYPOINT ["java", "-jar", "app.jar"]