Fix multi-arch Dockerfile COPY (v1.2.1 release failure)

[cjimti]

What broke

The v1.2.1 release failed at GoReleaser's dockers_v2 step:

docker images (v2): failed to publish artifacts: exit status 1
Seems like you tried to copy a file that is not available in the build context.
/tmp/goreleaserdocker...
├── Dockerfile
└── linux
    ├── amd64/rtbeat
    └── arm64/rtbeat

GoReleaser stages each binary under a ${TARGETPLATFORM} path in the multi-platform build context, but the Dockerfile did COPY rtbeat /rtbeat — nothing exists at the context root, so the build failed with "/rtbeat": not found.

Fix

Add ARG TARGETPLATFORM and copy ${TARGETPLATFORM}/rtbeat, matching the working kubefwd Dockerfile pattern. One file, one line of substance.

ARG TARGETPLATFORM
COPY ${TARGETPLATFORM}/rtbeat /rtbeat

Verification (locally, before asking for a re-tag)

• Reproduced the original failure: the old COPY rtbeat /rtbeat fails with "/rtbeat": not found; the fixed Dockerfile builds for both linux/amd64 and linux/arm64 via docker buildx.
• Ran the full GoReleaser pipeline in snapshot mode (release --snapshot): binaries, archives, Homebrew formula, and the multi-arch docker images all build successfully (release succeeded).

Not verifiable locally (CI-only, secrets required): the GHCR push, image cosign-signing, Homebrew-tap push, and SLSA provenance. GHCR login + packages: write already succeeded in the failed run, so these are the only residual unknowns.

After merge

The v1.2.1 tag points at the pre-fix commit (and the failed run left no release object or pushed image). Delete the stale v1.2.1 tag, then re-tag v1.2.1 on the fixed master.