CI improvements

.github/workflows/Dockerfile

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+FROM ubuntu:16.04 AS ubuntu-16.04-gcc-5
+# 🧰 Add basic tools.
+RUN apt-get update
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y software-properties-common python3-pip git
+# Note: We need to constrain the Pip version here because later Pip versions use f-strings internally which are only available in Python 3.6 and later but Ubuntu 16.04 only runs Python 3.5.
+RUN python3 -m pip install --upgrade pip~=20.3
+# 🧰 Install GCC compiler.
+RUN apt-get install -y gcc-5 g++-5
+ENV CC=gcc-5
+ENV CXX=g++-5
+# 🧰 Install CMake.
+RUN apt-get remove -y cmake
+RUN python3 -m pip install cmake~=3.24.0
+# 🧰 Install Doxygen.
+RUN DEBIAN_FRONTEND=noninteractive apt-get -y install doxygen
+
+
+# Note: GCC 8 is the latest available on Ubuntu 20.04
+FROM ubuntu:18.04 AS ubuntu-18.04-gcc-8
+# 🧰 Add basic tools.
+RUN apt-get update
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y software-properties-common python3-pip git
+RUN python3 -m pip install --upgrade pip
+# 🧰 Install GCC compiler.
+RUN apt-get install -y gcc-8 g++-8
+ENV CC=gcc-8
+ENV CXX=g++-8
+# 🧰 Install CMake.
+RUN apt-get remove -y cmake
+RUN python3 -m pip install cmake~=3.24.0
+# 🧰 Install Doxygen.
+RUN DEBIAN_FRONTEND=noninteractive apt-get -y install doxygen
+
+
+# Note: GCC 9 is the latest available on Ubuntu 20.04
+FROM ubuntu:20.04 AS ubuntu-20.04-gcc-9
+# 🧰 Add basic tools.
+RUN apt-get update
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y software-properties-common python3-pip git
+RUN python3 -m pip install --upgrade pip
+# 🧰 Install GCC compiler.
+RUN apt-get install -y gcc-9 g++-9
+ENV CC=gcc-9
+ENV CXX=g++-9
+# 🧰 Install CMake.
+RUN apt-get remove -y cmake
+RUN python3 -m pip install cmake~=3.24.0
+# 🧰 Install Doxygen.
+RUN DEBIAN_FRONTEND=noninteractive apt-get -y install doxygen