feat: add linux/riscv64 image

[ernado]

I want to add linux/riscv64 to supported architectures.

It is not in a first class port, same as already available linux/s390x.

However:

1. This port has no (and probably won't have, until it is first-class) available go binary downloads
2. The riscv64 port for Debian is unstable, so only available in sid.

Ref:

• x/website: add linux/riscv64 downloads golang/go#59113
• clarify Go support policy for secondary ports golang/go#53383

What I can do to make riscv64 images available?

[tianon]

Duplicate of #435 (which is still accurate).

[ernado]

Oh, thanks! Missed that one.

[ernado]

I will be maintaining a fork until official binaries are available and riscv64 is supported by debian.

Fork of image: https://github.com/go-riscv/docker-library-go
Binaries: https://github.com/go-riscv/go

Everything is built and released by github actions.

Also I'm happy to send invites to go-riscv if someone is interested.