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

Dockerfile_ARM

@@ -0,0 +1,4 @@
+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

@@ -0,0 +1,26 @@
+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
+ENV mkdir -p $HOME_DIR
+WORKDIR $HOME_DIR
+RUN git clone --recursive https://github.com/riscv/riscv-gnu-toolchain -j 8
+WORKDIR $RISCV_SRC_DIR
+# Checkout a specific hash to keep the output binaries consistent
+WORKDIR 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 8 
+# Remove the source code
+RUN 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

Dockerfile_X86

@@ -0,0 +1,3 @@
+FROM ubuntu:18.04
+RUN apt-get update 
+RUN apt-get install -y build-essential python