Added a dockerfile to build compiling environment to build X86, ARM, and RISCV binaries.

Dockerfile_ARM

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+FROM ubuntu:18.04
+RUN apt-get update 
+RUN apt-get install -y build-essential python \
+    gcc-arm-linux-gnueabi gcc-arm-linux-gnueabihf

Dockerfile_RISCV

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+FROM ubuntu:18.04
+RUN apt-get update 
+RUN apt-get install -y build-essential git curl gawk texinfo expat \
+    libexpat1-dev bison flex libz-dev python autoconf
+# Prepare to download and compile RISCV toolchain
+ENV HOME_DIR=/home/root
+ENV RISCV_SRC_DIR=$HOME_DIR/riscv-gnu-toolchain
+ENV RISCV_DIR=$HOME_DIR/riscv-tools
+RUN mkdir -p $HOME_DIR
+WORKDIR $HOME_DIR
+RUN git clone --recursive https://github.com/riscv/riscv-gnu-toolchain -j $(nproc)
+WORKDIR $RISCV_SRC_DIR
+# Checkout a specific hash to keep the output binaries consistent
+RUN git checkout 23a038856764808d75b6afe96f649980609ae4c6 
+# Downloading the prereqs must be done in riscv-gcc folder
+WORKDIR $RISCV_SRC_DIR/riscv-gcc 
+RUN ./contrib/download_prerequisites 
+WORKDIR $RISCV_SRC_DIR 
+RUN ./configure --prefix=$RISCV_DIR 
+RUN make -j $(nproc)
+# Remove the source code
+RUN rm -rf $RISCV_SRC_DIR 

Dockerfile_X86

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+FROM ubuntu:18.04
+RUN apt-get update 
+RUN apt-get install -y build-essential python