Use different cache for each image

.gitlab-ci.yml

-cache:
-  key: "$CI_COMMIT_REF_SLUG"
-  paths:
-    - build/
-  untracked: true
-
 stages:
   - build
   - test
@@ -17,6 +11,12 @@ build:gcc8-seq-coverage:
     - cd build
     - CXX=g++-8 CC=gcc-8 cmake .. -DCMAKE_BUILD_TYPE=Coverage
     - make pugs
+  cache:
+    key: "${CI_COMMIT_REF_SLUG}-gcc8"
+    paths:
+      - build/
+    untracked: true
+
 
 test:gcc8-seq-coverage:
   image: localhost:5000/ubuntu_gcc8
@@ -27,6 +27,11 @@ test:gcc8-seq-coverage:
     - cd build
     - make run_unit_tests
     - make coverage
+  cache:
+    key: "${CI_COMMIT_REF_SLUG}-gcc8"
+    paths:
+      - build/
+    untracked: true
 
 build:clang8-seq-coverage:
   image: localhost:5000/ubuntu_clang8
@@ -36,6 +41,11 @@ build:clang8-seq-coverage:
     - cd build
     - CXX=clang++-8 CC=clang-8 cmake .. -DCMAKE_BUILD_TYPE=Coverage -DCLANG_FORMAT=/usr/bin/clang-format-8
     - make pugs
+  cache:
+    key: "${CI_COMMIT_REF_SLUG}-clang8"
+    paths:
+      - build/
+    untracked: true
 
 test:clang8-seq-coverage:
   image: localhost:5000/ubuntu_clang8
@@ -46,3 +56,8 @@ test:clang8-seq-coverage:
     - cd build
     - make run_unit_tests
     - make coverage
+  cache:
+    key: "${CI_COMMIT_REF_SLUG}-clang8"
+    paths:
+      - build/
+    untracked: true