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/build

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# Build the image for one arch on that arch's daemon, logging to logs/DATE/.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+. bin/lib.sh
+
+arch="${1:?usage: bin/build <arch>}"
+use_arch_host "$arch"
+
+docker build --progress=plain -t "$(dev_tag "$arch")" . 2>&1 |
+    tee "$LOGDIR/build-$arch.log"