Add reference solution for exercise 1 in episode 6

* Reference solution for exericse 1 in episode 6 uses a stageless
pipeline and the interruptible keyword.

.gitlab-ci.yml

@@ -5,14 +5,8 @@
 
 ---
 
-stages:
-  - lint
-  - build
-  - test
-  - run
-
 default:
-  image: python:3.11
+  image: python:3.9
 
 .install_dependencies:
   before_script:
@@ -22,54 +16,55 @@ default:
 .compiler_versions:
   parallel:
     matrix:
-      - VERSION: [ "11", "12" ]
+      - VERSION: [ "10", "11", "12" ]
 
 license_compliance:
-  stage: lint
   before_script:
     - pip install --upgrade pip
     - pip install reuse
   script:
     - reuse lint
+  interruptible: true
 
 lint:
-  stage: lint
   before_script:
     - pip install --upgrade pip
     - pip install cpplint
   script:
     - cpplint --recursive src/ tests/
+  interruptible: true
 
 build:gcc:
+  image: gcc:${VERSION}
   extends:
     - .install_dependencies
     - .compiler_versions
-  image: gcc:${VERSION}
-  stage: build
   script:
     - cmake -S . -B build_gcc_${VERSION}
     - cmake --build build_gcc_${VERSION}
   artifacts:
     paths:
       - build_gcc_${VERSION}
+  needs: ["lint"]
+  interruptible: true
 
 test:gcc:
+  image: gcc:${VERSION}
   extends:
     - .install_dependencies
     - .compiler_versions
-  image: gcc:${VERSION}
-  stage: test
   script:
     - cd build_gcc_${VERSION} && ctest
-  dependencies:
-    - "build:gcc"
+  dependencies: ["build:gcc"]
+  needs: ["build:gcc"]
+  interruptible: true
 
 run:
-  image: gcc:12
-  stage: run
+  image: gcc:11
   script:
-    - ./build_gcc_12/bin/helloWorld
-  dependencies:
-    - "build:gcc"
+    - cd build_gcc_11/bin/ && ./helloWorld
+  dependencies: ["build:gcc"]
+  needs: ["build:gcc", "test:gcc"]
+  interruptible: true
 
 ...