add caching stage to docker build

Dockerfile

@@ -1,17 +1,21 @@
-FROM gradle:7.4.1-jdk17 as builder
+FROM gradle:7.4.1-jdk17 as cacher
 ENV APP_HOME=/usr/src/app
+ENV GRADLE_USER_HOME /cache
+WORKDIR $APP_HOME
+COPY build.gradle gradle.properties settings.gradle ./
+RUN gradle --no-daemon testClasses assemble \
+  # prevent errors from spotless b/c in cacher stage we have no source files yet
+  -x spotlessScala
+
+FROM gradle:7.4.1-jdk17 as builder
+ENV GRADLE_USER_HOME /cache
 WORKDIR $APP_HOME
+COPY --from=cacher /cache /cache
 COPY --chown=gradle:gradle . $APP_HOME
-# RUN ls -rtla $APP_HOME
 RUN gradle -v \
   && gradle testClasses assemble \
   && mv conf-jenkins.json conf-test.json
 
-FROM gradle:7.4.1-jdk17 as tester
-WORKDIR $APP_HOME
-COPY --from=builder . .
-RUN gradle test
-
 FROM openjdk:22-slim as prod
 WORKDIR $APP_HOME
 COPY --from=builder --chown=campudus:campudus ./build/libs/grud-backend-0.1.0-fat.jar ./tableaux-fat.jar