docker: publish branch-aligned repeater images and trigger rebuilds from core

[yellowcooln]

Summary

This PR makes the repeater Docker image build deterministic and branch-aligned with pyMC_core.

• dev repeater images build against pyMC_core@dev.
• main repeater images build against pyMC_core@main.
• The Docker publish workflow resolves the exact pyMC_core commit SHA and passes it into the image build.
• The Dockerfile installs pymc_core[hardware] from that exact SHA, then installs repeater without letting pip re-resolve pymc_core back to a cached branch tip.
• The RRD Docker fix from current dev is preserved, including librrd-dev, pkg-config, and the rrdtool Python dependency.

Workflow Behavior

• Push builds still run for main and dev, but only on pyMC-dev/pyMC_Repeater and yellowcooln/pyMC_Repeater.
• Fork/sync pushes in other repos no longer publish images automatically.
• Manual workflow_dispatch still works and lets a user provide a custom Docker image repository.
• repository_dispatch support was added so pyMC_core can trigger a repeater image rebuild with a known channel, core_ref, and core_sha.
• Upstream-only Home Assistant add-on notification remains limited to pyMC-dev/pyMC_Repeater.

Docker Compose / User Docs

• The example compose file now pulls pymcdev/pymc-repeater:dev instead of building locally.
• pull_policy: always is set so compose pulls the latest published image before recreate.
• The compose example now bind-mounts ./config to /etc/pymc_repeater and ./data to /var/lib/pymc_repeater.
• README setup steps now create ./config and ./data, run the radio config script against ./config, and document the default container UID/GID 15888 ownership requirement.

Notes

This PR adds the repeater-side dispatch receiver. The companion pyMC_core workflow and REPEATER_REPO_DISPATCH_TOKEN need to live in the pyMC_core repo separately.

Testing

• Rebased cleanly onto current pyMC-dev/dev.
• Built the Docker image locally with no PYMC_CORE_SHA provided, confirming the Dockerfile resolves pyMC_core@dev to an exact SHA.
• Ran an import smoke test inside the built image: rrdtool, pymc_core, and repeater import successfully.