Enable ccache and cache folder between builds.

.gitlab-ci.yml

 # This YAML recipe builds KStars with INDI support, then runs all tests
 
-# This is from packaging/linux/debian/control build-depends - fix that first if there is an issue in the CI build
-.install_build_dependencies: &install_build_dependencies
-  - apt update && apt -y install --no-install-recommends software-properties-common
-  - apt update && add-apt-repository ppa:mutlaqja/ppa
-  - >
-    apt update && apt -y install --no-install-recommends
-    gcc-multilib
-    g++-multilib
-    make
-    gettext
-    coreutils
-    cmake
-    extra-cmake-modules
-    qtbase5-dev
-    qtdeclarative5-dev
-    qtmultimedia5-dev
-    qtpositioning5-dev
-    libqt5websockets5-dev
-    libqt5svg5-dev
-    libqt5sql5-sqlite
-    kdoctools-dev
-    libkf5config-dev
-    libkf5guiaddons-dev
-    libkf5i18n-dev
-    libkf5newstuff-dev
-    libkf5notifications-dev
-    libkf5xmlgui-dev
-    libkf5plotting-dev
-    libkf5crash-dev
-    libkf5notifyconfig-dev
-    libeigen3-dev
-    zlib1g-dev
-    libcfitsio-dev
-    libnova-dev
-    libindi-dev
-    indi-bin
-    wcslib-dev
-    libraw-dev
-    libgsl-dev
-    phonon4qt5-backend-vlc
-    qt5keychain-dev
-    libsecret-1-dev
+# Build dependencies are in packaged into image definition 'docker/Dockerfile'
 
-# This is from packaging/linux/debian/control recommends/depends - fix that first if there is an issue in the CI build
-.install_test_dependencies: &install_test_dependencies
-  - apt update && apt -y install --no-install-recommends software-properties-common
-  - apt update && add-apt-repository ppa:mutlaqja/ppa && add-apt-repository ppa:pch/phd2
-  - >
-    apt update &&
-    apt -y --no-install-recommends install
-    make
-    cmake
-    extra-cmake-modules
-    xplanet
-    xplanet-images
-    astrometry.net
-    kded5
-    kinit
-    libindi1
-    indi-bin
-    breeze-icon-theme
-    libqt5sql5-sqlite
-    qml-module-qtquick-controls
-    phd2
-    xvfb
+# As of 202009, two-stage pipeline fails on propagation of artifact from stage build to stage test
+# Probably because the upload uses a webdav REST API, and there are many large files in the list
 
-#image: ubuntu:latest
+# We cache the ccache folder
+cache:
+  key: "$CI_JOB_ID"
+  paths:
+    - .ccache
 
 # Run the full validation in one step
 # The artifacts take far too much time to propagate from one step to the other
@@ -76,13 +19,16 @@ build-and-test:
   stage: test
   interruptible: true
   image: tallfurryman/kstars-ci
-  #  before_script:
-  #    - *install_build_dependencies
-  #    - *install_test_dependencies
+  before_script:
+    - apt update && apt install -y --no-install-recommends ccache
+    - mkdir -p .ccache
+    - export CCACHE_BASEDIR=${PWD}
+    - export CCACHE_DIR=${PWD}/.ccache
+    - du -hs ${CCACHE_DIR}
   script:
     - mkdir -p kstars-build
     - cd kstars-build
-    - cmake -DCMAKE_BUILD_TYPE=Release ..
+    - cmake -DCMAKE_BUILD_TYPE=Release .. -DCCACHE_SUPPORT=ON
     - make -j$(nproc) clean all install
     - rm -rf Testing
     - xvfb-run make test QT_TEST_TIMEOUT_FUNCTION=600000 QT_QPA_PLATFORM=eglfs ARGS='-T test -V --output-on-failure --no-compress-output' || true
@@ -93,59 +39,11 @@ build-and-test:
       junit:
         kstars-build/junit_result.xml
 
-# This part kept for reference, does not work on our gitlab instance
-# Mainly because of the propagation of artifact from stage build to stage test
-# Probably because the upload uses a webdav REST API, and there are many large files in the list
-# In the meantime, following up on the many gitlab issues reporting that problem
-
 # We build and test
 #stages:
 #  - build
 #  - test
 
-# We cache the build folder per job and per branch
-# cache:
-#   key: "$CI_COMMIT_REF_SLUG"
-#   paths:
-#     - kstars-build/
-
-# Consolidate runner with build packages and build
-#build-kstars:
-#  stage: build
-#  interruptible: true
-#  before_script:
-#    - *install_build_dependencies
-#  script:
-#    - mkdir -p kstars-build
-#    - cd kstars-build
-#    - cmake -DCMAKE_BUILD_TYPE=Release ..
-#    - make -j$(nproc) clean all
-#  artifacts:
-#    expire_in: 3600 sec
-#    paths:
-#      - kstars-build/
-#    exclude:
-#      - kstars-build/**/*.o
-
-# Consolidate runner with test packages and test
-#test-kstars:
-#  stage: test
-#  interruptible: true
-#  needs:
-#    - build-kstars
-#  dependencies:
-#    - build-kstars
-#  before_script:
-#    - *install_test_dependencies
-#  script:
-#    - cd kstars-build
-#    - rm -rf Testing
-#    - xvfb-run -a make test QT_TEST_TIMEOUT_FUNCTION=600000 ARGS='-T test --no-compress-output'
-#  artifacts:
-#    reports:
-#      junit:
-#        kstars-build/Testing/**.xml
-
 build-appimage:
   image: tallfurryman/kstars-ci
   before_script: