Remove redundancies from CI pipeline

* Remove redundancies from CI pipeline by using defaults, extends
and matrix keywords.

.gitlab-ci.yml

@@ -11,8 +11,20 @@ stages:
   - test
   - run
 
+default:
+  image: python:3.9
+
+.install_dependencies:
+  before_script:
+    - apt-get update && apt-get -qy install python3 python3-pip
+    - pip3 install cmake
+
+.compiler_versions:
+  parallel:
+    matrix:
+      - VERSION: [ "10", "11", "12" ]
+
 license_compliance:
-  image: python:3.11
   stage: lint
   before_script:
     - pip install --upgrade pip
@@ -21,7 +33,6 @@ license_compliance:
     - reuse lint
 
 lint:
-  image: python:3.11
   stage: lint
   before_script:
     - pip install --upgrade pip
@@ -29,60 +40,36 @@ lint:
   script:
     - cpplint --recursive src/ tests/
 
-build:gcc:11:
-  image: gcc:11
-  stage: build
-  before_script:
-    - apt-get update && apt-get -qy install python3 python3-pip
-    - pip3 install cmake
-  script:
-    - cmake -S . -B build
-    - cmake --build build
-  artifacts:
-    paths:
-      - build/
-
-build:gcc:12:
-  image: gcc:12
+build:gcc:
+  image: gcc:${VERSION}
   stage: build
-  before_script:
-    - apt-get update && apt-get -qy install python3 python3-pip
-    - pip3 install cmake
+  extends:
+    - .install_dependencies
+    - .compiler_versions
   script:
-    - cmake -S . -B build
-    - cmake --build build
+    - cmake -S . -B build_gcc_${VERSION}
+    - cmake --build build_gcc_${VERSION}
   artifacts:
     paths:
-      - build/
+      - build_gcc_${VERSION}
 
-test:gcc:11:
-  image: gcc:11
+test:gcc:
+  image: gcc:${VERSION}
   stage: test
-  before_script:
-    - apt-get update && apt-get -qy install python3 python3-pip
-    - pip3 install cmake
+  extends:
+    - .install_dependencies
+    - .compiler_versions
   script:
-    - cd build && ctest
+    - cd build_gcc_${VERSION} && ctest
   dependencies:
-    - build:gcc:11
-
-test:gcc:12:
-  image: gcc:12
-  stage: test
-  before_script:
-    - apt-get update && apt-get -qy install python3 python3-pip
-    - pip3 install cmake
-  script:
-    - cd build && ctest
-  dependencies:
-    - build:gcc:12
+    - "build:gcc"
 
 run:
-  image: gcc:12
+  image: gcc:11
   stage: run
   script:
-    - ./build/bin/helloWorld
+    - cd build_gcc_11/bin/ && ./helloWorld
   dependencies:
-    - build:gcc:12
+    - "build:gcc"
 
 ...