Support for RISC-V architecture (riscv64 Docker image)

[jplegat]

I tried to build a Docker image for riscv64 and noticed there are no binaries for it (CFG_CPUTYPE).

As more distributions are starting to officially support and increase adoption of riscv64 - in addition to the existing Docker support - it would be great to have a riscv64 Docker image option for those running RISC-V dev boards.

Thanks a lot for this project!