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/commit-logs

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Commit logs/DATE/ and push to origin.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+. bin/lib.sh
+
+git add "$LOGDIR"
+if git diff --cached --quiet; then
+    echo "no new logs to commit"
+    exit 0
+fi
+git commit -m "Release $DATE: build and push logs"
+git push origin HEAD