NOISSUE - use golang:1.26 builder to support riscv64 platform

[JeffMboya]

What type of PR is this?

Bug fix: multi-platform Docker build failing for riscv64.

What does this do?

• Switches builder from golang:1.26-bookworm to golang:1.26 (bookworm has no riscv64 manifest).
• Adds debian:sid-slim as the final stage for riscv64 (distroless/cc-debian12 has no riscv64 manifest).
• Sets CGO_ENABLED=0 for manager on riscv64 — wasmtime-go/v44 does not bundle a prebuilt libwasmtime.a for riscv64.
• Skips registry push on PR runs to avoid denied: installation not allowed to Write organization package.

Which issue(s) does this PR fix/relate to?

No related issue.

Have you included tests for your changes?

No tests needed; these are build and CI configuration changes only.

Did you document any new/modified features?

No documentation needed.

Notes

golang:1.26 defaults to bookworm for amd64/arm64, so runtime behaviour is unchanged there.

⚠️ Caveat: wasm plugin system is disabled on riscv64

The manager's wasm plugin feature relies on wasmtime-go/v44, which embeds prebuilt libwasmtime.a C libraries. Although wasmtime v44.0.0 itself ships a riscv64gc-linux-c-api release asset, wasmtime-go/v44 does not bundle it — the Go binding's build/ directory only includes:

Platform	Bundled in wasmtime-go
linux/amd64	✅
linux/arm64	✅
macos/amd64	✅
macos/arm64	✅
windows/amd64	✅
linux/riscv64	❌ (library exists upstream, not bundled)

References:

• wasmtime v44.0.0 release assets (riscv64gc-linux-c-api present): https://github.com/bytecodealliance/wasmtime/releases/tag/v44.0.0
• wasmtime-go/v44 build directory (no riscv64): https://github.com/bytecodealliance/wasmtime-go/tree/main/build
• wasmtime platform support: https://docs.wasmtime.dev/stability-platform-support.html

On riscv64, the manager builds with CGO_ENABLED=0 and any call to LoadWasm returns:
wasm plugin support requires CGO; rebuild with CGO_ENABLED=1

All other manager functionality (task scheduling, proplet management, MQTT, HTTP API) is unaffected.