Dockerfile: Install riscv32 toolchain from kernel.org

This is required to build GRUB UEFI target for RISC-V 32-bit.
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

Dockerfile

@@ -23,6 +23,7 @@ RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_
 RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_microblaze-linux.tar.xz | tar -C /opt -xJ
 RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_nios2-linux.tar.xz | tar -C /opt -xJ
 RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_powerpc-linux.tar.xz | tar -C /opt -xJ
+RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_riscv32-linux.tar.xz | tar -C /opt -xJ
 RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_riscv64-linux.tar.xz | tar -C /opt -xJ
 RUN wget -O - https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/7.3.0/x86_64-gcc-7.3.0-nolibc_sh2-linux.tar.xz | tar -C /opt -xJ