Add support for riscv64 architecture

[paralin]

Problem

Many architectures such as Arm and x86 are currently implemented here.

Multiple low-cost as well as high-performance riscv64 CPUs are now available.

Alpine, Ubuntu, have riscv64 images available in the Docker Hub library today.

Support for riscv64 should be added to docker-node as well.

Solution

Add riscv64 to the architectures list.

[nschonni]

RISC isn't support by Node right now nodejs/build#2815. The current discussions around getting that support are happening in nodejs/build#2876 and docker-node would wait for at least a sign that upstream is going to support it first.