Add RPi install docs, GHCR support, push-docker-all script

- README: Raspberry Pi install section with auto-detect architecture
- README: pre-built binary download section for all platforms
- Docker docs: dual registry (Gitea + GHCR)
- scripts/push-docker-all.sh: push to both registries in one command

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/push-docker-all.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# Build and push Docker image to both Gitea and GitHub Container Registry.
+#
+# Prerequisites:
+#   docker login git.manko.yoga        (Gitea — your username + token)
+#   docker login ghcr.io               (GitHub — your username + PAT with packages:write)
+#
+# Usage:
+#   ./scripts/push-docker-all.sh v0.6.0
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+if [[ -f .env ]]; then
+    set -a; source .env; set +a
+fi
+
+TAG="${1:?Usage: $0 <tag> (e.g. v0.6.0)}"
+
+GITEA_IMAGE="git.manko.yoga/manawenuz/btest-rs"
+GHCR_IMAGE="ghcr.io/manawenuz/btest-rs"
+
+echo "=== Building Docker image ==="
+docker build \
+    -t "${GITEA_IMAGE}:${TAG}" \
+    -t "${GITEA_IMAGE}:latest" \
+    -t "${GHCR_IMAGE}:${TAG}" \
+    -t "${GHCR_IMAGE}:latest" \
+    .
+
+echo ""
+echo "=== Pushing to Gitea ==="
+docker push "${GITEA_IMAGE}:${TAG}"
+docker push "${GITEA_IMAGE}:latest"
+
+echo ""
+echo "=== Pushing to GitHub Container Registry ==="
+docker push "${GHCR_IMAGE}:${TAG}"
+docker push "${GHCR_IMAGE}:latest"
+
+echo ""
+echo "Done! Images pushed:"
+echo "  ${GITEA_IMAGE}:${TAG}"
+echo "  ${GITEA_IMAGE}:latest"
+echo "  ${GHCR_IMAGE}:${TAG}"
+echo "  ${GHCR_IMAGE}:latest"
+echo ""
+echo "Pull with:"
+echo "  docker pull ${GHCR_IMAGE}:${TAG}"
+echo "  docker run --rm -p 2000:2000 -p 2001-2100:2001-2100/udp ${GHCR_IMAGE}:${TAG} -s -v"