Added caching of maven dependencies

backend/Dockerfile

@@ -1,9 +1,16 @@
 FROM maven:3.9-eclipse-temurin-21 AS build
 WORKDIR /app
+
+# Separate layer for dependencies: only re-runs when pom.xml changes.
+# The cache mount keeps ~/.m2 across builds so even pom.xml changes
+# don't require a full re-download.
 COPY pom.xml .
-RUN mvn dependency:go-offline -B
+RUN --mount=type=cache,target=/root/.m2 \
+    mvn dependency:go-offline -B -q
+
 COPY src ./src
-RUN mvn package -DskipTests -B
+RUN --mount=type=cache,target=/root/.m2 \
+    mvn package -DskipTests -B
 
 FROM eclipse-temurin:21-jre
 WORKDIR /deployments