dcache-build: use pom.xml file as key to cache maven dependencies

dcache-build.yml

@@ -12,7 +12,10 @@ variables:
 # Cache downloaded dependencies and plugins between builds.
 # To keep cache across branches add 'key: "$CI_JOB_NAME"'
 cache:
-  key: "$CI_JOB_NAME"
+  key:
+    files:
+      - pom.xml
+    prefix: "$CI_JOB_NAME"
   paths:
     - ./.m2/repository