updated runner to use different image

.gitlab-ci.yml

-image: ubuntu:18.04
+image: conanio/gcc9
 
 stages:
   - build
@@ -10,8 +10,8 @@ build:
     paths: 
       - code/bin
   script:
-    - apt-get update
-    - DEBIAN_FRONTEND=noninteractive apt-get install -y g++ cmake libopenmpi-dev libblas-dev liblapack-dev git libvtk7.1 ninja-build
+    - sudo apt-get update
+    - sudo DEBIAN_FRONTEND=noninteractive apt-get install -y libopenmpi-dev libblas-dev liblapack-dev git libvtk7-dev ninja-build
     - git submodule update --init --recursive
     - cd code/build/debug
     - cmake -G Ninja -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTING=True -DCMAKE_EXPORT_COMPILE_COMMANDS=True ../../

README.md

@@ -10,8 +10,8 @@ TBD
 
 ## Build
 ### Required dependencies
- - Compiler with C++20 support
- - cmake >= v3.5
+ - Compiler with C++20 support (e.g. g++ v9)
+ - cmake >= v3.12.4
  - LAPACK
  - OpenMP
  - MPI
@@ -24,8 +24,7 @@ Note that an **active internet connection is required for the first build** in o
 For the first build only, download all submodules:
 
 ```bash
-git submodule init
-git submodule update
+git submodule update --init --recursive
 ```
 
 ### Compile the code

code/CMakeLists.txt

-cmake_minimum_required( VERSION 3.5 )
+cmake_minimum_required( VERSION 3.12.4 )
 project( RTSN LANGUAGES CXX VERSION 0.0.1 )
 set( CMAKE_CXX_STANDARD 20 )
 set( CMAKE_CXX_STANDARD_REQUIRED ON )