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/native-arch

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# Print this machine's arch in docker naming (amd64 or arm64).
+case "$(uname -m)" in
+    x86_64 | amd64) echo amd64 ;;
+    aarch64 | arm64) echo arm64 ;;
+    *) echo "unsupported arch: $(uname -m)" >&2; exit 1 ;;
+esac