use our own Docker image for CI jobs

.gitlab-ci.yml

 default:
   tags: [ public-docker ]
-  image: easybuilders/test:centos7-eb4.2.1
+  image: $CI_REGISTRY/easybuild-docker/easybuild-docker/centos8-eb-test:latest
 
 check_style:
-  before_script:
-    - pip3 install --user pep8
-    - pip3 install --user easybuild
   script:
     - export SYSTEMNAME="juwelsbooster" # Make this the default for packages like NVHPC
-    - export PATH="$HOME/.local/bin:$PATH"
-    - export PYTHONPATH="$HOME/.local/lib/python3.6/site-packages"
     - export EASYBUILD_INCLUDE_EASYBLOCKS="$CI_PROJECT_DIR/Custom_EasyBlocks/*.py,$CI_PROJECT_DIR/Custom_EasyBlocks/generic/*.py"
     - export EASYBUILD_INCLUDE_MODULE_NAMING_SCHEMES="$CI_PROJECT_DIR/Custom_MNS/*.py"
     - export EASYBUILD_INCLUDE_TOOLCHAINS="$CI_PROJECT_DIR/Custom_Toolchains/*.py,$CI_PROJECT_DIR/Custom_Toolchains/fft/*.py,$CI_PROJECT_DIR/Custom_Toolchains/compiler/*.py,$CI_PROJECT_DIR/Custom_Toolchains/linalg/*.py"