Docker: Reduced Dockerfile for running builds on GHActions.

The reduced Docker image only contains GCC and other
open-source tools but no commercial ones.
diff --git a/docker/dockerfile.gnu b/docker/dockerfile.gnu
new file mode 100644
index 0000000..c0fc072
--- /dev/null
+++ b/docker/dockerfile.gnu
@@ -0,0 +1,66 @@
+# Due to bandwidth limitation, we need to keep the base image into our
+# Artifactory Docker Registry. Because we have more than one registry,
+# we need to set during build time which Artifactory Docker Registry to use.
+ARG DOCKER_REGISTRY
+FROM ${DOCKER_REGISTRY}/ubuntu:focal
+
+# install packages from official Ubuntu repo
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && \
+    apt-get install --no-install-recommends -y \
+        bc \
+        build-essential \
+        curl \
+        dos2unix \
+        git \
+        lib32stdc++6 \
+        mscgen \
+        p7zip-full \
+        python3 \
+        python3-pip \
+        tar \
+        unzip \
+        wget \
+        libxml2-utils \
+        zip && \
+    apt-get autoremove -y && \
+    apt-get autoclean -y && \
+    rm -rf /var/lib/apt/lists/*
+
+# Create build ARGs for installer files & versions
+ARG GCC=gcc-arm-none-eabi-10-2020-q4-major-x86_64-linux.tar.bz2
+
+# Including dependency folder
+ARG DEPENDENCIESFOLDER=dependenciesFiles
+ARG TOOLS_PATH=/opt
+ARG INSTALLER_PATH=/tmp/dependenciesFiles
+RUN mkdir -p ${INSTALLER_PATH}
+COPY dependenciesFiles/${GCC} ${INSTALLER_PATH}/${GCC}
+COPY dependenciesFiles/buildtools ${TOOLS_PATH}/buildtools
+COPY dependenciesFiles/python-matrix-runner ${INSTALLER_PATH}/python-matrix-runner
+
+# install & setup gcc
+RUN mkdir -p ${TOOLS_PATH}
+WORKDIR ${TOOLS_PATH}
+RUN tar -xvf ${INSTALLER_PATH}/${GCC}
+ENV PATH=${PATH}:${TOOLS_PATH}/gcc-arm-none-eabi-10-2020-q4-major/bin
+ENV CI_GCC_TOOLCHAIN_ROOT=${TOOLS_PATH}/gcc-arm-none-eabi-10-2020-q4-major/bin
+WORKDIR /
+
+# install Python requirements
+COPY requirements.txt ${INSTALLER_PATH}/
+RUN python3 -m pip install --no-cache-dir -r ${INSTALLER_PATH}/requirements.txt
+
+# install buildtools
+RUN python3 -m pip install --no-cache-dir -r ${TOOLS_PATH}/buildtools/requirements.txt
+COPY rtebuild /root/.rtebuild
+ENV PATH=${PATH}:${TOOLS_PATH}/buildtools
+
+# install python-matrix-runner
+# hadolint disable=DL3013
+RUN python3 -m pip install ${INSTALLER_PATH}/python-matrix-runner
+
+# remove dependency folder
+RUN rm -rf ${INSTALLER_PATH}
+
+CMD ["bash"]
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