Add per-brach per-job caches to build jobs

.gitlab-ci.yml

@@ -24,6 +24,10 @@ build:main:
       $DUNECONTROL --only=dorie all
     - MAKE_FLAGS="$MAKE_FLAGS"
       $DUNECONTROL --only=dorie make build_tests
+  cache:
+    key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
+    paths:
+      - $CI_PROJECT_DIR/build-cmake
   artifacts:
     name: "$CI_JOB_NAME"
     paths:
@@ -36,12 +40,15 @@ build:update_dune:
     - master
   script:
     - $DUNECONTROL update
-    - $DUNECONTROL exec "rm -rf build-cmake"
     - CMAKE_FLAGS="$CMAKE_FLAGS"
       MAKE_FLAGS="$MAKE_FLAGS"
       $DUNECONTROL --module=dorie all
     - MAKE_FLAGS="$MAKE_FLAGS"
       $DUNECONTROL --only=dorie make build_tests
+  cache:
+    key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
+    paths:
+      - $CI_PROJECT_DIR/build-cmake
   artifacts:
     name: "$CI_JOB_NAME"
     paths:
@@ -60,6 +67,10 @@ build:debug:
       $DUNECONTROL --only=dorie all
     - MAKE_FLAGS="$MAKE_FLAGS"
       $DUNECONTROL --only=dorie make build_tests
+  cache:
+    key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
+    paths:
+      - $CI_PROJECT_DIR/build-cmake
 
 test:exec_cmds:
   stage: test