Add linux/riscv64 to official release binaries

[gounthar]

Description

Docker CLI already has linux/riscv64 in its _platforms variable for cross-compilation, but riscv64 is not included in official release artifacts.

I maintain docker-for-riscv64, a complete Docker stack for RISC-V that has shipped Docker CLI riscv64 binaries across 117+ releases (see releases). Docker CLI runs without issues on native riscv64 hardware (BananaPi F3, SpacemiT K1, rv64gc, Debian Trixie).

Since the cross-compilation support is already in place, this is about including riscv64 in the release workflow. I'd be happy to submit a PR and can provide native build logs, test results, and ongoing testing on real hardware.

All other Docker stack components are gaining riscv64 support upstream — BuildKit already cross-compiles for riscv64, Compose has it in binary-cross, and issues are open on runc, containerd, BuildX, and Moby.