GitLab CI: Define cache, Maven options.

Part-of: <https://gitlab.benkard.de/mulk/jgvariant/-/merge_requests/19>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a478c90..3f0dba2 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -11,5 +11,14 @@
 sast:
   stage: check
 
+cache:
+  key: $CI_JOB_NAME
+  paths:
+    - .m2/repository
+
+variables:
+  MAVEN_OPTS: "-Dmaven.repo.local=.m2/repository"
+  MAVEN_CLI_OPTS: "--batch-mode --errors"
+
 include:
   - template: Security/SAST.gitlab-ci.yml