Update .gitlab-ci.yml

.gitlab-ci.yml

 image: gradle:jdk12
 
 before_script:
-  - export GRADLE_USER_HOME=`pwd`/.gradle
+  - export GRADLE_USER_HOME=`pwd`/lectures/.gradle
 
 cache:
   paths:
-    - .gradle/wrapper
-    - .gradle/caches
-    - build
+    - lectures/.gradle/wrapper
+    - lectures/.gradle/caches
+    - lectures/build
 
 build:
   stage: build
   script:
-    - gradle build
+    - cd lectures
+    - ./gradle build