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

Dockerfile

@@ -0,0 +1,27 @@
+FROM ubuntu:18.04
+RUN apt-get update \
+    # Install gcc for x86 and ARM
+    && apt-get install -y build-essential gcc-arm-linux-gnueabi gcc-arm-linux-gnueabihf git \
+    curl gawk texinfo expat libexpat1-dev bison flex libz-dev python \
+    # Prepare to download and compile RISCV toolchain
+    && export HOME_DIR=/home/root \
+    && export RISCV_SRC_DIR=$HOME_DIR/riscv-gnu-toolchain \
+    && export RISCV_DIR=$HOME_DIR/riscv-tools \
+    && mkdir $HOME_DIR \
+    && cd $HOME_DIR \
+    && git clone --recursive https://github.com/riscv/riscv-gnu-toolchain -j 8\
+    && cd $RISCV_SRC_DIR \
+    # Checkout a specific hash to keep the output binaries consistent
+    && git checkout 23a038856764808d75b6afe96f649980609ae4c6 \
+    # Downloading the prereqs must be done in riscv-gcc folder
+    && cd $RISCV_SRC_DIR/riscv-gcc \
+    && ./contrib/download_prerequisites \
+    && cd $RISCV_SRC_DIR \
+    && ./configure --prefix=$RISCV_DIR \
+    && make -j 32 \
+    # Remove the source code
+    && rm -rf $RISCV_SRC_DIR 
+
+# For x86: gcc
+# For ARM: arm-linux-gnueabi-gcc or arm-linux-gnueabihf-gcc ?
+# For RISCV: $RISCV_DIR/bin/riscv64-unknown-elf-gcc