Add Makefile and bin/ scripts for one-command multi-arch release

make release builds, smoke-tests and pushes amd64 (on fsn1app1 via
DOCKER_HOST) and arm64 (local), joins them into latest and dated
multi-arch manifests, and commits the day's logs. Logic lives in bin/;
the Makefile only wires dependencies so -j2 runs the arches in parallel.

bin/manifest

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# Join the per-arch tags into multi-arch manifests IMAGE:latest and
+# IMAGE:DATE, and record what was published.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+. bin/lib.sh
+unset DOCKER_HOST
+
+refs=()
+for arch in ${ARCHES:-amd64 arm64}; do
+    refs+=("$(arch_tag "$arch")")
+done
+
+docker buildx imagetools create \
+    -t "$IMAGE:latest" -t "$IMAGE:$DATE" "${refs[@]}"
+
+docker buildx imagetools inspect "$IMAGE:$DATE" | tee "$LOGDIR/manifest.txt"