Add `riscv64` Architecture Support for Docker Image

[rodneyosodo]

Problem Statement

Currently, the official authzed/spicedb Docker image does not include support for the riscv64 architecture. This limits the ability to run SpiceDB on newer RISC-V platforms and development boards that are increasingly being adopted in edge and research environments.

Solution Brainstorm

• Add riscv64 as a target platform in the build pipeline (e.g., using docker buildx or ko multi-arch builds).
• Optionally, provide experimental images until CI/CD runners with riscv64 become stable.

Use Case

• Running SpiceDB on RISC-V edge devices for confidential or decentralised authorisation setups.
• Testing multi-architecture orchestration setups using Docker or Kubernetes on RISC-V clusters.

[tstirrat15]

Gonna reopen this - see the comment here: #2601 (comment)